Started adding support for managing own shared templates.

So far, a user is only able to manage the templates shared with him, but he can't manage the templates he shared with others. The shared template dialog will allow this as well.
Added methods to query the database for templates shared by the user.
Completed implementation of "sharing by" templates.
Changed wording of notifications.
Cleaned template action icons.
Merge branch 'feature/template_sharing_extended' into feature/template_sharing