Add support for twitter feeds with auto-posting similar to boards

[jace]

Just like boards can be user-defined and can have custom auto posting rules, Hasjob needs user-specified Twitter feeds. These feeds should retweet from the main feed (and not post fresh).

To make this work:

1. [ ] The JobPost model should store tweet references in the model itself
2. [ ] When a post is tweeted, the tweet URL/id should be stored in the model
3. [ ] A new TweetFeed model and associated models should mirror the Board model and others
4. [x] Hasjob needs to be a registered app with Twitter, separate from Lastuser (it is already, with posting access)
5. [ ] Setting up a TweetFeed should lead the user to authenticate with Twitter, then save their credentials to the TweetFeed model, with the user's choice of auto-posting options. Feeds may or may not be linked to a board
6. [ ] When a post goes live and the auto-tag helper kicks in to decide which boards it needs to be posted to, the same is done for tweet feeds as well. Each tweet is pushed into a new background job

[jace]

#465 introduces tweet ids stored inside the JobPost model.