Retweet instead of Tweet

[jace]

Hasjob's "Tweet this" feature used to be used a fair bit. It's barely used now. To revitalise this mode of sharing, Hasjob should:

1. Keep track of tweets made by the background job,
2. Embed this tweet on the job description page, and
3. Encourage a retweet from here.

[jace]

Related to #323.