v0.63.0

Moving pages into a handbook is one bulk action now, "Move into a handbook…", with the handbook chosen in a dropdown that appears beside it. Before this every handbook was its own entry in the bulk menu, which grew with each handbook a site created and read as a list of unrelated actions rather than one action with a target.

An empty choice is refused by the browser's own validation, the server checks as well and says so if nothing was moved, and the action is not offered at all while there is no handbook to move into.

Nothing needs doing on update.