add (composable) filters for projects

[mattxwang]

With #57 on the horizon, we are going to have a lot of projects. It might get hard to sift through as the end user, which is where a filtering system would help!

The simplest filter to implement is probably off of language. I would suggest starting there; to implement it, I would ask that you:

1. Create a UI element (it's okay if the styling isn't perfect)
2. Devise some mechanism to perform the filtering - noting that it could get more complicated once we add more filters
3. Add some sort of state to manage this
4. Optional: save the result of the filter to the URL, so you can "send" someone a filter with a link

I would suggest you either branch off of #57 or wait for it to resolve, since it changes the language structure.