Clientside error logging

Add support for client-side error logging using sentry. Uses the jquery and native plugins to catch exceptions in normal javascript, and adds an Angular exception handler (via a decorator) to catch angular errors.

Two new settings: 

  sentry.src - can be either a local js file or remote url (e.g. getsentry cdn)
  sentry.publicDSN - the public sentry token

If sentry.src is not set, or sentry is not loaded (either because it is turned off or fails to load) then all of the functionality is inactive.