Pull local theme into separate package

[kmpaul]

As mentioned by @dcamron in #115, the new Portal theme is "local," meaning that it is contained in a separate directory in this repository, as opposed to being in its own repository (e.g., ProjectPythia/sphinx-pythia-theme). @dcamron suggested:

Would we look to pull the local theme back out to its repo after some time?

I believe the answer to this is "yes" and with the merging of #115, I am holding @dcamron's suggestion here in this issue until it gets resolved.

[kmpaul]

With ProjectPythia/sphinx-pythia-theme#12, I believe we will be able to retool the Portal to use the external theme. I will do this in another PR (that should close this issue when merged).