Re-enable client crash reporting for the extension

The browser extension currently writes Sentry settings for the sidebar app under the [`raven` key](https://github.com/hypothesis/browser-extension/blob/41a58a393cbf62869ff3f87d900d02dd1c85ffa6/tools/settings.js#L60) but the client looks for them under the [`sentry` key](https://github.com/hypothesis/client/blob/56bd5077edf2b2b2f03d570221066eb2a8fcc080/src/sidebar/index.js#L24). 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/.