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.
Instead of calling /sources.json and rendering the "Fetch source data"
box client-side in Javascript, call /sources.js so we can render the
html server-side.
Fixes#3807. Drops the TagScript module entirely and replaces it with
a simple call to Post.tag.
Simultaneously fixes#3773 as a consequence of removing TagScript.run.
Post.update already updates data-tags by itself, so there was no need
for TagScript.run to update data-tags as well.
