Deprecate use of obsolete link.allow_liveupdate

[manishapme]

There's only 19 rows in postgres for the allow_live_update key and it hasn't been written to in a year. Think it's safe to deprecate.

[spladug]

This is an anti-evil safeguard that predates the is_hidden_discussion checks added much later below. It's intended to allow an employee to remove links from the sidebar on livethreads because they're abusive/trolly. If you're going to remove this option, which is fine because contributors have their own more flexible powers now, it'd be good to make sure the existing hidden threads are moved over to the new system so people viewing old live threads don't see the nasty stuff that was deliberately hidden.

pedantic nit: deleting the attribute is removing it. deprecation is when it's still around but you're saying "please don't use it for new stuff"