Add 503 code to HTTP headers helper
Return 503 error when the account is currently locked.

There are some cases where the account will be locked because we are
migrating its data (such as upgrading for hashtags). We have defined
this as one of the potential errors API clients might run into when
working with the API.
Added links to each API resource in our navigation.

This hasn't been styled at all, but it makes it easier to browse the
API on a browser (for when you're reviewing it).