Add check to make sure we don't already have an existing (current) installation - before we get to prereq. stage.

 Template is used for canned auto-reply - triggered by filters.

This will remove the upper limit of BLOB sizes imposed by MySQL with the
max_allowed_packet setting completely.

This adds a new table %file_chunk which will contain the filedata in smaller
chunks (256kB). It also includes a new class, AttachmentChunkedData, which
will handle reading and writing the data, abstracting away the chunks.

This is done by migrating data from the %file table to the %file_chunk
table. One must beware that this must safely (the migration that is) plug
into the both the live osTicket developers as well as the users doing a full
upgrade from osTicket-1.6*. For this, the AttachmentFile::save method was
patched to use the new AttachmentChunkedData class to write the attachment
data to the database in chunks. That is, the migrater will use the new code
on the major upgrade and bypass the filedata column of the %file table
altogether. Therefore, the patch associated with this commit will not
migrate any data for the major upgrade.

For developers doing incremental upgrades, the patch included in this
commit will transfer the data from the %file data to the new %file_chunk
table by chunking it. As written, only the first 16MB of the attachment is
migrated. This could easily be adjusted, but it seems like a reasonable
limit for now.

Force target selection on filter form
Add minor code improvements

* Change table name from ost_email_filter* to ost_filter*
* Add patch for target field.

And rearrange files to work with the new layout for the finalized upgrader

The upgrader can't (and shouldn't) do everything necessary to complete an
upgrade. The adminsitrator will need to take care of a few extra tasks that
are outlined in the UPGRADING.txt file. One of the items includes removing
source code no longer used in the 1.7 codebase. A script is shipped (Unix
only) that allows an administrator to automatically clean up all old source
files that are no longer used.