Fix an exception that is triggered by mousing out of a thumbnail when
tooltips are disabled. Caused by trying to access `qtip.cache` when
`qtip` is null.
* Cancel pending ajax requests when mousing out of the thumbnail.
Prevents multiple requests from piling up if the user moves in and out
of the thumbnail before the first request completes. This normally
isn't possible except during slowbooru.
* Show an error message if the ajax request fails unexpectedly.
Refactor things to store information about the current user as data
attributes on the <body> tag rather than as <meta> tags. These <meta>
tags are now deprecated and will be eventually removed.
* Store all of the current user's API attributes as data attributes on
the <body> tag.
* Add `CurrentUser.data` for getting data from the <body> tag, and
use it instead of `Utility.meta`.
* Add `CurrentUser.update` for updating the current user's settings.
* Fix a bug with the user named "Anonymous" not being able to edit notes.
