Retroactive auto-tagging of watched words

Specifying auto-tag action for watched words is effective for topics created after the specification. It would be very useful to retro-actively auto-tag all past topics as well.

This functionality could go in cutomize->watched words->tags, where the auto-tag action is specified. Might also be natural on tags page → hamburger pull-down (function that would apply all auto-tag actions on all topics).