remote: Be robust against bad names

If we use a remote name containing questionable characters
such as newlines or '[', we will run into assertions in
GKeyFile. To avoid that, check that the group name we
pass is valid, and throw an error otherwise.

Found while writing tests.

Closes: #2244
Approved by: alexlarsson
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f