Move column initializers into __init__ calls.

Several of the columns directly assigned member variables that ought to have
been included in __init__ instead. While this was perhaps slightly more
readable, it meant that the Column.__init__ method couldn't operate on them.

This change moves all those parameters into the __init__ calls. It also splits
the "To Me" column into using detailed_label_html and detailed_label, to
dovetail with my djblets change that fixes the sort order in the column
selector.

Testing done:
Looked at the column selector and saw that everything was in alphabetical
order, even when the column included HTML or other entities at the beginning of
the text.

Fixes bug 3260.

Reviewed at https://reviews.reviewboard.org/r/5538/
Make a bunch of updates to the user manual.

This change updates a lot of the text in our docs to match the 2.0 reality
better. This doesn't yet update any of the images--I'll go through and take a
bunch of new screenshots in a later change. There are also some larger changes
that I'd like to make to the content, which I'll be filing issues about to
track.

This includes one small change to a label on the profile page for accounts,
since when I was writing the docs, I realized that some installs may hide the
fields that are referenced by "Keep this information private".

Testing done: Built HTML docs and read through them.

Reviewed at https://reviews.reviewboard.org/r/5527/
Add registration and extension support for custom auth backends.

The auth backend code now follows the same registration pattern used
throughout the codebase. Auth backends can be registered, unregistered,
iterated through, and looked up.

A new requirement is that auth backends must have an ID. For
compatibility purposes, we assign an ID if one is not set when scanning
entry points, but all registrations going straight through the new
functions will require an ID set directly on the class.

There's also a new AuthBackendHook for extensions, which will handle the
registration and unregistration for the extension.

Documentation has been added for the new hooks, and the guide on writing
an auth backend has been updated as well.