Process inline attachments in thread entry and support inline images in
piped emails

Support inline images across the system, with draft support

Migrate to a single attachment table
    That way we don't need a new table for everything we need to attach an
    inline image to (like a signature, for instance)

Add richtext support for internal notes

Implement images on site pages

* Image paste in Redactor
* Make non-local images optional
* Placeholder for non-local images
* Fix local image download hover
* Don't re-attach inline images

This is one of the last major hurdles to make osTicket fully translatable.
With the advent of 1.7.0, database migration is based on the MD5 hash of the
install SQL file. Therefore, the install SQL script cannot be modified (ie.
translated) or else the system cannot be migrated, since the MD5 hash will
no longer match up.

This patch pulls initial data out of the install file so that it can be
translatable. Eventually the install worksheet can allow a drop-down box of
the default system language. The default data from that language will be
loaded into the system.

SLAs can be marked transient. When a ticket is assigned to a transient SLA
and it is transferred to a department or help topic having a default SLA,
the SLA will change to the SLA of the new department or help topic. This
process can continue as long as the ticket has a transient SLA assigned.
Once a non-transient SLA is assigned to the ticket, the SLA will no longer
change automatically. Thereafter, the SLA can only be manually changed.