Make Navigate to Editor more discoverable

[jcansdale]

What this PR does

This PR gives users a hint that the Navigate to Editor functionality exists.

It adds the following messages:

1. When a PR file diff gets focus
   

2. When a PR file diff gets focus from a PR that isn't checked out
   

3. When user attempts to navigate to a file that isn't checked out
   

4. When the diff file focus is lost the status is cleared (showing the default Ready message)
   

• This also works when using View File to view a (read-only) PR file

Reviewers

• What do you think of the wording? Is my alternative suggestion above an improvement?
• Do you think the status bar message is noticeable enough without being overly distracting?

Todo

• Finalize the wording. Would something like this be better?
  • Press Enter to navigate to Editor (PR branch must be checked out)
• ~~It would be nice if the dialog gave Checkout and Cancel options~~~
  • Maybe do this in a separate PR

Part of #1491
Fixes #1546

[grokys]

LGTM!

[meaghanlewis]

Have been dogfooding this and I think the message is helpful. It's not at all distracting, because the bar is always there- I'm just used of it only saying Ready.