Modernize the guides on creating extensions.

Review Request #188 — Created Dec. 1, 2016 and updated — Latest diff uploaded

Review Board
This fleshes out the various pages on creating extensions. There's now a
new, fully-detailed walkthrough of the extension class and the
attributes/methods that need to be used. The guides to the file/package
layout and packaging have been updated to talk about Wheels, static
media bundling, dependencies, etc. The configuration and static media
pages go into more details on the options available, along with
examples. The database models page now talks about writing evolution

The resulting documentation is now a lot more clear, helping truly guide
extension authors to getting started. It should be easier for us to
continue to maintain and update as we go forward.

Testing Done:
Built the docs and went through them, checking for spelling errors (though
it's possible I missed some), links, source code, etc.

(Some links are currently broken, just because there are some new pages that
aren't part of this change.)

Reviewed at