[ticket/11915] Improve Plupload integration

[prototech]

Changes that this implements:

• Replaces the current Plupload jQuery widget with our own custom uploader.
• All files are now consolidated into the same table under the attachments tab and are handled by the new uploader. This means that files that have been previewed/submitted can also be deleted without a page reload.
• Errors are displayed using phpBB's alert handler.
• The inline attachment bbcodes in the message are now updated on the client-side after a file is uploaded/deleted.
• Files start uploading immediately after they're selected. The "Start upload" button is no longer used.
• The uploader now checks for trigger_error() being called and displays the appropriate message to the user.
• The user can now provide a file comment or place the file inline immediately after it's uploaded.
• A link to view the file is now provided to the user, just as is the case when previewing.
• The drag-and-drop zone is now the message textarea.

http://tracker.phpbb.com/browse/PHPBB3-11915
http://tracker.phpbb.com/browse/PHPBB3-11961

[prototech]

Updated. No longer WIP.

[EXreaction]

Drag and drop doesn't seem to work for me?

[EXreaction]

@cyberalien Could you take a look at this code?

[EXreaction]

I didn't see any issues with the code when I looked at it last time and @cyberalien seems to have disappeared. If you resolve the conflicts I'll merge this.