Revision history UX

### As a content moderator editing, I don't want a draft that is a duplicate of a live page to save/display, so that I'm not confused/chasing down a ton of revisions that are all the same.

Acceptance criteria:
When a user hits "save draft" but the draft is the same as what's live:
- [ ] on the listings page, the page still says "live" (not "live + draft")
- [ ] on revision history, there isn't a new revision logged for this (non-)change. (In other words, we should never see a revision that, when you compare drafts, says "There are no differences between these two revisions)

**Is your feature request related to a problem? Please describe.**
In #2099, we outlined the problem that Joplin saves drafts when no changes have been made, which causes previously "live" pages to then be listed as "live + draft," even though the draft is an exact duplicate of the live. And then when you go to the revision history and compare the drafts, it says "There are no differences between these two revisions."


**Describe the solution you'd like**
Make it so that Joplin is smart enough to know that a saved draft is a duplicate and not have it display as a new revision or "live + draft." This includes when users select "preview" or "share draft."

**Describe alternatives you've considered**

**Additional context**