"Tagged journal entries" block for group/institution/site journals

Spinning this off from Bug 1540156. The "tagged journal entries" block is only available for user pages, not for group pages, individual pages, or site pages. But it should be available in all those places, now that groups, institutions, and the site can have their own journals.

However, this will not be a trivial thing to implement. The problem is that don't have an equivalent to the "user_tags" table for groups, institutions, or the site. So, the block has no easy way to know what tags exist on artefacts owned by those entities.

It's not an insurmountable challenge, but it does mean it'll take a bit of effort to do this.