Resolve "Mark as viewed stays checked in Single File Mode"

What does this MR do?

For #299948 (closed)

Switches to using the Vuex as the source of file reviews, not relying on a single fetch out of localStorage when the component loads.

This fixes numerous issues with the "Viewed" checkbox on files when it's used in the single-file view mode.

MR	Description
We are here 👉🏻	Because we don't always render a new Diff File component, incorporating file reviews with a "pull" method doesn't work, we need a "push" method a la Vuex subscriptions.
Future MR	We don't need to store reviews nested under their file_identifier_hash, since each ID incorporates the file and the MR, we'll simplify the review storage by just storing the review information associated to the file ID at the top level.

Screenshots (strongly suggested)

For these scenarios, we trigger a full page refresh, then perform the recorded actions.

Scenario	Notes	Before	After
"Typical", check the viewed box, then move to the next (unviewed) file	Here note that in the before, the check persisted when moving to the next file, but the collapsed state didn't	beforeTypical	afterTypical
Where the initial load is of a "Viewed" file, and we move to a file that is not marked as viewed	Here note that it has the same behavior as the previous scenario - the check incorrectly persists to the next file	beforeViewedToNot	afterViewedToNot
Where the initial load is of a file that is not "Viewed", and we move to a file that is viewed	Here, note that neither the check nor the collapsed state updates properly when moving between files with different states	beforeNotToViewed	afterNotToViewed
Where the initial load is of a "Viewed" file, then we uncheck it, and move to the next - not viewed - file	Note here that the uncheck incorrectly flows into the next file, which should be checked (and collapsed)	beforeUncheckViewedToViewed	afterUncheckViewedToViewed

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