Since those strings can be consumed by the PausedDebuggerOverlay, on the server,
we need to put them in devtools/shared.
As this will create some work for the l10n team, we migrate them to Fluent so we're
feeding 2 birds with 1 scone.
The <LocalizationProvider> is added directly in <WhyPaused> (and not in <App>,
as it's messing up with the React context (See Bug 1743155).
Some snapshots are updated. It's unfortunate that we don't see the translated
strings there anymore, but we can't have nice things in Jest.

While working around those strings, I realized we didn't
check the text of the component in the different situations
it handles, so I added a few assertions.

Depends on D132260

Backed out for causing mochitest failures on browser_metrics_debugger.js.

Backout link: https://hg.mozilla.org/integration/autoland/rev/c6b7e343048bd01cfd8f5df563795debcd558fdf

Push with failures : https://treeherder.mozilla.org/jobs?repo=autoland&resultStatus=testfailed%2Cbusted%2Cexception%2Crunnable&revision=48c568e6d4d4497f7886264efb3fe93346f4341f&selectedTaskRun=MHXJVi03SdWDvm5ABGftPQ.0

Link to failure log : https://treeherder.mozilla.org/logviewer?job_id=359928129&repo=autoland&lineNumber=2865

Failure message: TEST-UNEXPECTED-FAIL | devtools/client/framework/test/metrics/browser_metrics_debugger.js | Duplicated module load detected. List of duplicated modules

https://hg.mozilla.org/mozilla-central/rev/6eaac86a4922
https://hg.mozilla.org/mozilla-central/rev/41993db6c7a1