Added post note dragging and resizing in frontend
Changed notes to be rendered only in image posts
Removed fileinfo dependency

Recent changes in thumbnail generators revealed that finfo module cannot
be relied on - I found a GIF file that started with usual "GIF87a", and
yet finfo reported its mime type to be "application/octet-stream".

This change removes reliance on finfo module and implements detecting
common file types supported by szurubooru in manual fashion. This might,
of course, introduce some bugs that weren't present when using finfo.
However, this change was made exactly because of such bug - while
previously such edge cases were impossible to fix, now they can be
dealt with accordingly in our own code.

Also from what I saw on the Internet, getting fileinfo to work on
Windows is a lot of trouble, so this change is welcome from that angle,
too.
Fixed NetworkingService not using MimeHelper
Added note CRUD to backend
Fixed typo in post feature confirmation prompt
Extracted post history services
Fixed post snapshots being non-deterministic
Fixed file dropper appearance
Fixed file dropper triggering form submit
Added post note editing to frontend
Fixed undeclared variable in HomePresenter
Added support for nested snapshot elements
Added post note information to post snapshots
Updated TODO
Improved image posts scaling
Fixed post notes drag boundaries
Changed post notes to work with %-based coords
Fixed post note edit form position for big posts