Display Loading Overlay when posting a response or a note to a ticket.

Can not update list item abbreviation because update() method is
expecting 'abbrev' key so this corrects the key.

* HTTP Option: TRUSTED_PROXIES (default: <none>
To support running osTicket installation on a web servers that sit behind a
load balancer, HTTP cache, or other intermediary (reverse) proxy; it's
necessary to define trusted proxies to protect against forged http headers.

* HTTP Option: LOCAL_NETWORKS (default: 127.0.0.0/24)
When running osTicket as part of a cluster it might become necessary to
white list local/virtual networks that can bypass some authentication
checks.

* Validate CLIENT_IP to make sure it's a valid IP address.

Encode html chars on helpdesk title

Defer to agent's timezone on due date `datetime` selection instead of
defaulting to GMT.

Fix case where filter applying 'Use Reply-To Email' filter action
triggers a reprocessing loop that exhausts php memory.

Fixes missing due date on task create caused by invalid datetime format sent to db.

Use class instead of id on form element that triggers overlay on submit.

Address cases where incoming email doesn't have message-id or references

Allow closed thread to be reopened when when is passive mode

Add support for threading email conversations happening outside the system.
Autoresponders are paused when threading is on passive mode. Alerts may
still go out to agents as configured.

Account for agents' name format setting when sorting agents.

MessageThreadEntry::add vs MessageThreadEntry::create

db_real_escape expects a string

Add department relationship to Canned model.

Fix bug brought about by commit #72e9abcf

When issue summary is disabled for a specific help topic, the system uses
help topic name as the subject line.

Due to validation error on disabled subject field the setValue fails to take
in the help topic. The change here allows for forced value set.

Refactor PR #3410 to drop extra commits.
All credit to @jdelhome3578

Remove extra ` ` which throw off the spacing

This fixes sorting and total ticket count.

The ThreadEntry::normalizeFileInfo() method is called from ::create(), but
before the thread entry is saved. Therefore, the ::getThread() and
::getObject() methods will likely not return a valid object for posting the
thread message.

The [signal documentation]
(https://github.com/osTicket/osTicket/blob/develop/setup/doc/signals.md)
for the `cron` signal states that the context is `null`. But it was
missing until now (and the `$data` parameter was wrongly forwarded as
`$object`)