Enable Sentry integration for client

[robertknight]

hypothesis/client#1320 integrated the new Sentry JS SDK into the Hypothesis client to capture details of crashes and report them to us automatically. In order to benefit from this in the context of the browser extension we need to add the necessary configuration to the settings/chrome-{prod, stage}.json config files.

I would suggest that we use the same Sentry project for the Hypothesis client across all delivery methods (via, embed, extensions) so that reports are combined together. However we can use the environment config option to differentiate between reports from dev, qa and prod environments and to differentiate between reports from the embedded client vs. the Chrome extension.

We can either use the same DSN as for the embed or create a new DSN specifically for the extensions.

Note that this issue is separate from enabling Sentry for the code that runs in the privileged context of the extension's background page.

[robertknight]

See also #745.