Re-enable client crash reporting for the extension

[robertknight]

The browser extension currently writes Sentry settings for the sidebar app under the raven key but the client looks for them under the sentry key. As a result, we are not receiving Sentry reports from the sidebar app loaded in the client.

When we fix this, we'll also need to add the origins (chrome-extension://{ID}, etc.) of our extensions to the Sentry origin allowlist under the Client Security list at https://sentry.io/settings/hypothesis/projects/client/.