Create intersphinx modeling to store in database

[ericholscher]

We should be capturing the metadata that is embedded in the Intersphinx data in each build. There are a number of features that we could build on top of this, but the first step is to start tracking it in a database.

Uses

We have an existing example of how to use this data in our search indexing PR here: readthedocs/readthedocs-sphinx-ext#48.

Also could be used with #1812.

It would also be neat to be able to link to rtfd.org/projects/docs/intersphinx/cname-ssl and have it redirect to the proper URL, for cases like #4766.

[ericholscher]

Have a working version of this locally. Now playing with integrations to build on top.

[agjohnson]

I feel like this might benefit from some more discussion before making this priority work. We've skipped a few steps. These features don't seem like strong features for the platform, however using domain data to populate our indexes would be extremely helpful.

I'm assuming we need to figure out the search upgrade problems before that is possible though.