Add base.html, create blocks for template abstraction

[dgmouris]

Issue: #36

I've added quite a few blocks, but I can change this if need be I just thought some of these would be handy in the future:)

[ethanchewy]

Overall, good job!

I'm going to leave this open for the meantime while I rethink about what features I want to add onto PythonBuddy and how to format the templating.

For example, I think for the base file, we should include any of the code content blocks because for future pages like when a user logs in, views their previous submissions, they won't need the code block.