Feature: Ability to attach files to issues (attachments)

This fixes #304.

Add the ability to add attachments to issues/comments.
A new button was added below the text area of comments/issues, which allows the upload of multiple files.

Files get listed below the text of an issue/comment. Hovering over the files gives a little preview (for images).
If an issue/a comment gets deleted the files attached get deleted too.

There are 4 new config values that controll the uploads. (under [attachment])