Tag list should display a scroll bar for accessibility

The tag list is scrollable but never displays a scrollbar of any kind. For me this is an accessibility problem because I have great difficulty using a scroll wheel or scroll gesture. I often rely on clicking and dragging a scrollbar to scroll things. I'm using macOS 11.1 and I have the option "Show scroll bars" set to "Always" in _System Preferences_ > _General_.

I get that there's not a lot of room there, which I am assuming is part of the reasoning for not having a scroll bar there. Perhaps it could be displayed on hover to make it less obtrusive (even though it contradicts the system preference above).

### Expected
I expected the tag list to display a scrollbar.

### Observed
It's possible to scroll the tag list with a scroll wheel but not with a mouse button.

### Reproduced
You need enough tags in the tag list so that they overflow and require scrolling. I suppose you could also set your System Preferences as I mentioned above and then open the app.


System Information:
- Experienced on both an 5k iMac and a MacBook Pro
- macOS 11.1 Big Sur
- Simplenote 2.6
- _System Preferences_ > _General_ > _Show scroll bars: Always_

No file or test is identified. Reproduce the problem in Simplenote for macOS with enough tags to overflow the list and macOS set to show scroll bars always; the work is done when the tag list provides a mouse-draggable scrollbar while preserving the available space.

Written by the indexing model from the issue text.