imap: subscriptions support

[emersion]

[protobits]

I'm trying hydroxide with Thunderbird and I noticed that I can only access a folder, after which clicking on other folders does not work. It appears Thunderbird opens multiple connections to the server in these cases and hydroxide does not seem to support it. I'm mentioning this here since I'm not sure if this is "subscription support" or it is actually a more general issue of multiple connections.