Support abbreviations for tree names / validate tree names

[robarnold]

m-c -> mozilla-central

We need to canonicalize the name in the UI code though so this should be exposed as some function which maps strings to string or undefined.

[sdwilsh]

I think builds.js is only going to support the path after hg.mozilla.org/, but you are welcome to make that mapping function for the UI!

[robarnold]

I'll let you fill out the database since you know the trees better than I do.

[sdwilsh]

I only know the main trees, but I'm filling out kTrees in builds.js right now.