Tag Management: Cover image upload

[ErisDS]

One of the major features of the new tag management screen is the ability to make the tag archive pages look a little bit nicer by adding a cover image.

This should work exactly the same as the existing implementation on the post settings menu, with the more recent improvements to make sure it resets properly.

This could potentially be split out into a reusable component for the post and tag settings menu, as well as any other similar menus we add in future.

The result of this issue should be a working dropzone on the tag settings menu, which stores the uploaded file path in the image property of the tag model and resets correctly. It should automatically work with the {{image}} helper.