Allow for adding or removing a Space Watch on behalf on another user

Confluence administrators (globally) and spaces administrators (for their spaces) should be able to add or remove a space watch on behalf of another user. This is key in teams where new people come onboard and the team leader wants them to keep up to date with what's going on without waiting for them to learn about how notifications work in Confluence.

Within the UI, this can be done in "Manage Watchers" by adding this feature to the "Watching this space" as is exists for "Watching this page" (instead of "You cannot modify this list").

It needs to be added to the API as well, as watchSpace will only work for the authenticated user.

N.B. this is NOT a duplicate of CONF-5866 as there is still no way to add or remove a space watch for another user.