Resolves #3220 - show heartbeat status in top bar

[alexcottner]

Resolves #3220 . Show the heartbeat endpoint status in top bar. Status icon is also a link that will take the user to the __heartbeat__ endpoint. Also resolves a brief error that users see during redirect to auth.

• Add check to top bar (first draft done)
• Add unit tests
• Fill in missing unit tests (don't have tests for SessionInfoBar yet)

Screenshots of error (red !) vs success (green circle):

[grahamalama]

Thanks for updating this to use Redux + Sagas. I spun up this issue for us to track the conversation for adopting new state management / data fetching patterns.

For this PR:

• I see you elected to add a new heartbeat slice of the store. Any reason you didn't opt too add a property in the session slice, e.g. session.heartbeat? We are adding this to the SessionInfoBar, after all
• Adding a few saga tests (see these examples for session sagas) might be a good idea

[alexcottner]

I see you elected to add a new heartbeat slice of the store. Any reason you didn't opt too add a property in the session slice, e.g. session.heartbeat?

Mainly because the session slice is written to localStorage, and that felt a little awkward. Do you think that's a reasonable place for it though?

[alexcottner]

Added a few tests for the saga.

[grahamalama]

Do you think that's a reasonable place for it though?

It does feel more reasonable to have this in the session slice, but yeah, doesn't make sense to save to local storage. Maybe we could update saveSession / loadSession to account for that.

However, seeing as this is blocking a Remote Settings release. Maybe we can call this PR good for now and create a follow on issue.

I think all we need to call this PR done is 1 test that demonstrates that the heartbeat is re-fetched after the timeout winds down.