When the H client is served from a local URL (eg.
chrome-extension://ID/scripts/foo.bundle.js), the original
source URLs and source maps are not accessible
to Sentry.

Enabling source maps for extensions is done in three steps,
the first two of which are implemented in this commit:

 1. Changes to the crash reporting to transform local URLs
    into filenames when reporting exceptions.

    This results in URLs for source files in Sentry
    reports which are independent of where the files
    are served from
    (eg. chrome-extension://ID, moz-extension://ID...)

 2. A Gulp task that uploads JS bundles and their associated
    sourcemaps to Sentry using the Sentry API.

    The assets are uploaded to Sentry using just the filename as
    the URL, so the source files and source maps are correctly
    matched up with the transformed URLs produced by the changes
    in (1).

 3. Configuring CI builds to run the Gulp task from step (2).

This commit makes use of ES2015 template strings in
upload-to-sentry.js, so the JSHint file has been adjusted
accordingly.