Add author, timestamp, and history per-block

One feature I really like in a couple of other apps is a visible author and timestamp for each block, showing both creation and modification date. A further enhancement to this is to have full per-block history. Notion implements the former, while Quip implements the latter.

Notion’s author and timestamps, shown with on-hover black popup window.

Quip shown without a block selected (no author/history displayed):

Quip with block selected for editing (click into, just like Anytype). Note timestamp and author now displayed on left:

Quip shown after clicking author/timestamp, displaying popup on left with full history. Clicking an entry in the block history allows you to revert changes per-block:

Some of this may be overkill, especially the full per-block history (maybe provide an option to limit the depth of per-block history or disable it entirely?). But the basic timestamp and author stamp are vital in my opinion.