Create/update Mercurial bookmarks from the online interface

It would be useful to be able to bookmark a commit in a mercurial repo from the web interface. Equivalent to:

#!bash

hg bookmark -r <commit-id> <bookmark-name>

The interface could be similar to the "add a tag" function, except that it should give a list of existing bookmarks to update as well as the option to type in a new bookmark name.

This is a particular issue because:

• Bitbucket pipelines currently do not work on tags in Mercurial, only on bookmarks (see BCLOUD-13819)
• Commits created using the online editor cannot currently be set to activate and advance an existing bookmark (and if you try to activate a bookmark by selecting it from the "branches" list for the commit, you end up creating a branch by that name, see BCLOUD-7361 / BCLOUD-8154)
• Merges created from the online editor likewise do not update bookmarks on the target (see BCLOUD-12287)

Of course, this would still be a useful enhancement even if those bugs/limitations were fixed!