model: remove UI notification feature

This commit is part of the removal of the UI notification feature from
Kallithea, which is not deemed useful in its current form. Only email
notifications are preserved.

docs: cleanup of casing, markup and spacing of headings

Mostly stuff found and fixed by Søren, extracted here to separate things.

Other uses of title casing might be debatable, but here it was just a few
documentation headings that clearly were inconsistent with the majority.

First step in two-part process to rename directories to kallithea.
This first step is to change all references in the files where they refer
to the old directory name.