Re-add hook just removed; compose templates are so scattered, there is nowhere a...

Re-add hook just removed; compose templates are so scattered, there is nowhere a plugin can add output to the bottom of the compose page except to output its own template on this hook -- must make sure plugins NEVER output directly here without using the template API (it is possible a plugin could slip this by us).  Steve did his best with converting the compose page to templates, but it's a pretty big mess, with template chunks and still a lot of output coming directly from compose.php, especially things like the form tags and misc formatting - this must be cleaned out.

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@12345 7612ce4b-ef26-0410-bec9-ea0150e637f0