Issue Summary 

This is reproducible on Data Center: yes

After disabling SSH at a server level, it's no longer possible to add SSH keys, making it impossible to add an SSH key for commit signing. It is however possible to use an SSH key for commit signing that was added before disabling SSH. 

Steps to Reproduce

1. Disable SSH at a server level
2. Attempt to add an SSH key intended for use as a signing key.

Expected Results

Either of the two below:

• SSH keys can still be added with SSH disabled at a server level. SSH connections would still be disabled, but users would still be allowed to sign commits with the SSH key that was added.
• The key can't be added and SSH key commit signing is disabled for keys that exist in the system already

Actual Results

• The SSH key cannot be added to the account and only existing keys can be used for SSH commit signing

Workaround

Enable SSH at a server level, add the SSH key to the account and then disable SSH at a server level.