UI to replace source's annotations of label A with B

[StephenChan]

For example, replace all 'Pavement' annotations in a particular source with 'Noncoral' annotations. The Noncoral label should already be added to the source's labelset in order for this to work.

We already have a Django management command for this - see PR #290. We then said on the Google Group once or twice that we'd take people's requests to run this management command on their sources. However, it'd be better to let source admins run this function themselves somehow.

I think the best place for this function would be the Browse Patches page, since this allows for further filtering of the annotations if desired; for example, one could run this only on Unconfirmed annotations, or only on annotations done in June 2021 by a particular user. So you'd apply the filters, then there'd be an action box at the bottom of the page which lets you perform an action on the selected annotations - for now, the only available action would be changing the selected annotations to a particular label. Since this could be a very large batch action which could be very difficult to reverse, there should be an 'are you sure' confirmation dialog.

If you remember issue #44, which we've thought of doing many times over the years, there is definitely some overlap there. Essentially, right now I'm proposing adding the 'assign label' action, without bothering with any of the other ideas for the Browse Patches page yet: