put schema docs up on GitHub Pages

[chadwhitacre]

Reticketed from #1320. SchemaSpy seems to work well for this:

http://schemaspy.sourceforge.net/sample/

Except that they put ads on the pages. :-( We'll have to rip those out.

Ideal here would be to set up a webhook so that anytime schema.sql changes on master (which is supposed to happen only after the DDL has changed in production) we rebuild the schema docs (and rip out the ads).

[zbynekwinkler]

@whit537 I would rather keep it part of #1312.

[chadwhitacre]

Agreed, @zwn. Let's close this in favor of #1272, which is about more hand-written docs on the db schema (per #1502).