A button to bulk lock issues and PRs #635

This is similar to #633 but I want a button to lock threads rather than unsubscribe. I really want the ability to lock all closed threads and PRs that have been closed for more than a few weeks. The submissions on old closed threads are usually poor. If users really have issues with old PRs I would prefer to push them towards opening new issues if there's a regression or some other problem. Following up to the closed PR is bad workflow in the case of an otherwise good submission. In many cases, though, following up to closed PRs and Issues is just users with the same exception, but a completely different cause, piling into an old PR or Issue creating noise. Its classified incorrectly, it derails the old thread, and its not in a position where its actionable.

It'd be nice to be able to search on locked and unlocked status on the issues webui search page. It'd be nice to see the locked/unlocked status as an icon. And it'd be nice to be able to bulk lock and unlock by page and by all the filter results. Similar to #633 it would also be nice to be able to automate this via the API and I expect there you need the same feature of being able to get the Thread ID of the thread in order to click the lock button via the API (I have not investigated this, however).