switch user lookup to API v2

[cloutier]

So I'm working on porting Birdsite to the v2 of the twitter API, because they are reticent about giving me v1 access. I got quite carried away in my branch with lots of changes and I realized the PR would be quite gnarly. So I'm splitting it up with user lookup being the first part. The code is in a working state at this commit and uses both v1 and v2 for different parts.

The only thing I found so far that is missing from v1 to v2 is user banners. I use the same image in both profile and banner and it looks okay, but I guess I could just remove the banner entirely.

I have lots of things coming up in other PRs, including having retweet broadcast as proper Announce:

Let me know what you think about this, I'm happy to do any changes!