Admin server list: a connect link and an enabled switch per server (#1035)

[lucamzanon]

#1035 adds two optional fields to the admin server list, and since it touches the admin area I'd rather ask before you spend time on it.

• Connect link: when the selected server has one, the login form shows "No account on {server} yet? Connect one". This is for servers where accounts are created somewhere else.
• Enabled: an unchecked server is hidden from the login form and refused server-side, without deleting its configuration. Handy to switch a server off for a while.

Existing configs keep working unchanged, and the link is not shown for a server that has its own OAuth sign-in.

Do you want both, one of them, or neither? I'll trim or close the PR based on your answer.