Get all dynamic fields in DynamicFormEntry, not just fields with answers

This partially reverts commit bff191b6.
The hasSpecialSearch() method can be retired in `develop-next` — NOT in the
`develop` branch

This allows for multiple priority fields to specify differing defaults, and
it also allows for a selection of 'System Default' in the config, which
renders as 'Default' when rendered.

It also fixes up a couple PHP warnings about include/class.forms.php

If the ticket details form has an extra field of type "Priority Level"
(beyond the one built in), exports of tickets will be empty.

This patch addresses the issue in the exporter which uses an older version
of the custom data materialized view (__cdata table), which created separate
columns for selection and ID values. The current cdata system only creates a
single column and stores the ID value. It also addresses an issue where the
ID column was passed to the PriorityField::to_php() as the first argument.

This script adds a single download script, 'file.php', which provides access
to files of all types to all users. It uses a HMAC signature system with an
expires time, which allows signed URLs to be sent to external users.

This also fixes an issue with the Http::cacheable() method, where the
last-modified and Etag headers were not properly compared, which resulted in
permanent cache misses by the client.

Ensure that when generating the list of acceptable file extensions, that the list is lower cased, because the extension from the filename will be lower-cased before attempting to find the extension in the list of acceptable extensions.

Previously, this was not ported properly now that multiple selections are
supported in the choice field and derivatives.

There was a slight issue when transitioning from a short answer field to a
drop-down field, and when the CDATA table is dropped and rebuilt.

Determine and set a reasonable max file size on install
Remove file extension restrictions - transition to mime types

This patch fixes a small glitch where validation might be bypassed for
fields for agents. Previously there was a glitch where required fields would
have their validation errors ignored.

This allows administrators to select a default value for selection fields
deriving from Custom Lists. This has been implemented on the ChoiceField,
but was forgotten on the SelectionField

  * Use the title of "Information" with internal tag of "info"
  * Add "Visibility" column to UI and retire "Required" and "Internal"
    columns
  * Add visibility selection to new fields and show visibility to the user
    information fields when displayed on the ticket details form
  * Make the visibility settings translatable
  * Use constants internally for visibility configuration detection
  * Use a larger input when managing the information HTML content
  * Fix validation errors for new fields
    * Enforce new field name uniqueness
    * Enforce new field name character validation

This is similar to the section break but doesn't have the <hr> element
before it and allows for HTML input (including images)

Hide multi selection option for typeahead selections

This will allow administrator to indicate if end users are allowed to reopen
tickets in a particular closed status.

Hiding a field triggers visibility check on dependent fields
Use jQuery specific value getters

This is necessary to avoid confusion vs. closed state.

Also fix crash if the [name] of a widget element started with a digit
character.

This allows administrators to define a regular expression to filter content
entered into short answer fields by agents and endusers. With the new
visibility feature, the regular expression input is only shown if the
regular expression validator is selected.

This allows fields to specify a visibility constraint that will be evaluated
in real time in the browser to automatically show and hide fields that
should be hidden based on values of other fields in the same form.

Validation is not performed on fields server-side if they are considered
invisible when submitted.

Furthermore, revamp the __cdata system to better support multiple selection
fields. Drop the `.._id` fields and use a coalesce of the ID value of the
entry value or the entry value itself. For JSON backed fields, strip some
JSON characters to result in a comma-separated list.

The search engine now uses MySQL FIND_IN_SET to analyze a search selection
in advanced searches. Also, interaction with the MySQL full text search
backend is fully supported.

Not just the SelectionField. This will make room for the InlineFormField