implement vanity URLs

Nobody has claimed this yet.

Now that we have the API server talking to Consul we can do pretty much whatever we want for vanity URLs. Off the top of my head, I like the idea of having users pick an "account name" and using that as a slug in the first part of the URL. Basically:

Old: `https://cdn.livepeer.com/hls/q6h6uwjf3oe63liv/index.m3u8`
New: `https://cdn.livepeer.com/livepeer/iameli/index.m3u8`

Rough implementation:
1. New field on `user` called `slug`. In my example, it would be set to `livepeer`.
1. New database table and API endpoints for `/api/vanity-url`. Fields: `{id, streamId, slug}`. For my example stream, my `slug` would be "iameli".
1. When a new stream activates (`POST /api/stream/:id/setactive`) check to see if that streamId has a `vanity-url`. If so, create a Consul route like so:
```
/{user.slug}/{vanityUrl.slug}/* --> /hls/{stream.id}/*
```
Simple as that.

Start with the API server's user and stream handling, the database schema, and the POST /api/stream/:id/setactive path. Trace how Consul routes are created, then define the user slug and /api/vanity-url data and endpoints. Done means an active stream with a vanity URL creates the specified /{user.slug}/{vanityUrl.slug}/* route to /hls/{stream.id}/*.

Written by the indexing model from the issue text.