[RFR] Background jobs

For different reasons, we need to run things in the background:

* Change SSH keys
* Delete branches
* *later* Edit a file
* *later* Clone a remote

For this reason, I bootstraped a simple bundle to handle background jobs.

Many things must still be addressed:

- [x] Signal handling in shell
- [x] Test branch delete feature
- [x] Implement SSH Key as background job
- [x] Technical documentation for background jobs
- [x] Credentials before displaying button
- [x] Credentials on controller call