Git authorization with dedicated SSH key

[akondas]

This is follow up from #309 and #45: Support is needed to authorize the repository with a regular ssh key. I would see it as a separate tab where you can add your keys, and then on the package adding card it would be possible to select and authenticate a given key. Here is a simple visualization of how I imagine it:

Acceptance criteria:

• [ ] user can add own key
• [ ] user can generate a new key pair (and download public key)
• [ ] on git type repository user can select available key
• [ ] repman will synchronize package using provided key
• [ ] user can rotate selected key (optional)