Allow for the use of multiple ssh keys

**Is your feature request related to a problem? Please describe.**
Github only allows a deploy key to be used once, so effectively you can only use weblate with a single github repository (unless you're using https credentials).

**Describe the solution you'd like**
Allow for using multiple SSH keys with weblate, to enable one-per repository/project.

**Describe alternatives you've considered**
HTTPS is workable here but it involves creating spoof deploy accounts which is far from ideal.