Add project proposal to repo

[dideler]

As suggested by @jshyeung.

Currently we host the master version on GDocs and have students copy it, modify the permissions, then send us the link. This workflow usually has lots of annoying issues.

Instead we'll have students work on their proposal in a topic branch and send us a PR (which won't get merged, but it makes it easier to find all the proposals). This also gives them extra Git & GitHub experience at the start.

[mtomwing]

We could even make a repo just for proposals and give all students push access.

[dideler]

👍 Keeps it out of the codebase. There's no harm in giving them push access, but I think they should still submit PRs (they just won't have to fork the repo).

[dideler]

It'd be interesting to see how that repo evolves if we don't touch it at all. Simply give the students team admin access to that repo and have them take care of it all. We should still review their PRs though, but we wouldn't be in charge of merging them.