roomList: Add "Disconnect" to network context popover

It can be useful to temporarily disable an account - for instance a work network while being on vacation - but we lost the ability to do so from Polari itself when dropping the separate connections dialog in commit 263ff952. Re-expose the option by adding a "Disconnect" action to the network context popover that mirrors the existing "Connect" one, and let the actions control the account's :enabled property to preserve the connection state over restarts.
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passed #6384