Possible to edit note in Preview mode

### Expected
It should be impossible to edit a note when in preview mode.

### Observed
There are at least two ways to edit a note when in preview mode.
1. Open edit mode, focus the tag input and hit `Cmd` + `Shift` + `Y`
2. Select a note in preview mode via note list and hit `Tab`

`1` is in the video:

https://user-images.githubusercontent.com/73365754/119486844-5f1ac780-bd61-11eb-89ea-829bd4d2c149.mov

### Reproduced
Scenario 1:
1. Open a note and activate preview mode
2. Set focus to tags input
3. Press `Cmd` + `Shift` + `Y` ~> this will bring the focus to note editor (still in preview mode), and it will be possible to enter text.

Scenario 2:
1. Open a note and activate preview mode
2. Select this note in the note list
3. Press `Tab` ~> you can enter text

Make|Model|macOS Version|App Version
-|-|-|-
Apple|MBP 16"|11.2.3|2.13.6

Start by reproducing both keyboard sequences in the macOS app and compare focus behavior between preview mode and edit mode. Trace the focus and keyboard handling reached by the tag input, note list selection, and Tab key; done means neither sequence allows text entry while preview mode is active.

Written by the indexing model from the issue text.