Added the ability to upload multiple files for an assignment.

When creating an assignment, there is an option to allow the learners to upload multiple files, and if this is selected the admin can set a maximum number of files. Because of this, submissions with multiple files are downloaded as .zip archives, and when downloading all submissions at once the individual student submissions are put into folders within the archive. The commenting system had to be revised as well, and now comments are only attached to the assignment and the learner's proxy_id, not the individual afile_id's.
Moved multiple assignment files sql upgrade to new db version folder.
Merge branch 'develop' of https://github.com/EntradaProject/entrada-1x into feature-assignment-files

Conflicts:
	www-root/core/modules/admin/gradebook/assignments/add.inc.php
	www-root/core/modules/admin/gradebook/assignments/edit.inc.php
	www-root/core/modules/admin/gradebook/assignments/grade.inc.php
Fixed a few bugs in queries for assignment files/comments.
Updated the install SQL, which was missing the 1627 database changes.
Replaced multiple ob_end_clean with ob_clear_open_buffers
A number of UI refinements to adding / editing assignments; Replaced usages of mktime() function with time().
Merge branch 'feature-assignment-files' of github.com:EntradaProject/entrada-1x into feature-assignment-files
Merge branch 'develop' into feature-assignment-files
Fixed a few minor bugs in the assignments section of the gradebook module, and the profile. Also added in the missing `notice_id` field to the `assignments` table which is used by the assignment notices feature.
Merge pull request #461 from EntradaProject/feature-assignment-files

Reviewed the changes and fixed a few things I found, but overall everything looked good to me, and Matt didn't break any of my notices stuff, but I found something (the sql change added to the 1627 upgrade script) I had missed in that feature branch- so there's that!