Automate BridgeDB documentation builds

[isis]

The developer documentation for BridgeDB needs a place to live. Currently, Sphinx builds can be manually triggered to produce HTML documentation (as well as other types). I have been manually scp'ing it to ​https://para.noid.cat/bridgedb, mostly because I had no idea where to put it, but wanted to put it somewhere.

So...

1. We should probably ask TPO's sysadmins to create something like ​https://docs.torproject.org/bridgedb, or wherever people want to use for keeping common developer documentation.

2. I think there was some sort of funding for this. I've just been doing this because I needed to understand how BridgeDB works and it was entirely undocumented when I started working on it. If someone knows who that funder is, please comment/tag this ticket as appropriate.

3. Documentation builds should probably be triggered automatically when a new signed tag is pushed to git.torproject.org/bridgedb.git.

[isis]

It's not exactly automated, but in my fix/12507-automate-documentation-builds branch, I added some commands to the make docs Makefile directive which ultimately produce a bridgedb-$VERSION-docs.zip file. This file can be uploaded through PyPI's maintainer interface to produce the hosted version at ​https://pythonhosted.com/bridgedb/.

[cass]

Removing SponsorZ tag because it looks like the issue has mostly been resolved. Please re-add tag if this is not the case and/or it becomes an active issue that needs new funding.

[gaba]

Remove sponsor 19 and add a keyword ex-sponsor19 to mark all the tickets that could have been in the scope of the sponsor.