Adds support for custom token TTL via the authenticate endpoint and HTML form

[ajres]

fixes #1847

---

This change is

[tleyden]

The unit tests failed with this error:

https://gist.github.com/tleyden/4d6ae597c0d0cbb5f5a6f156a59ff4a7

Going to re-run

[tleyden]

Re-running, failed with the same error. @ajres can you run the full test suite locally and see if it's passing for you?

You can see the test params here: https://github.com/couchbase/sync_gateway/blob/master/.drone.yml

./test.sh -race -cpu 4

[tleyden]

It doesn't look like the unit test failure could have anything to do with this change though .. I wonder if we should just merge it? @adamcfraser any thoughts?

[adamcfraser]

I haven't seen that failure running locally. It looks like the usual 'bucket is closed before db is stopped' error. In any case it looks unrelated to this change - I think it's ok to merge.