Replace Resque with GoodJob (and Redis with Postgres)

[mediafinger]

Idea

Have one (or even two) dependencies less and trust in Postgres for:

• background queue handling as ActiveJob backend
• cron-like functionality

See:

• https://github.com/bensheldon/good_job

TODOs when actually using Jobs

• setup any background job (e.g. email sending)
• prepare cron like setup
• write specs for both