Fix help usage

LPA enhanced their stageprompt tracking. They now add to the event label the source of event (which page the help was clicked, example `stageprompt.lpa:help.inline:create/lpa-type:what-if-i-want-to-make-more-than-one-lpa`).

This required a change in the backdrop ga collector to handle multiple values in a field: https://github.com/alphagov/backdrop-ga-collector/pull/14

Limelight had to change in a few places:
* They want `help.inline`, not `help` usage.
* Use `eventDestination`, instead of full `eventLabel`, for description.

**WARNING: Can only deploy after LPA release their changes and collectors are updated. Coordinate with Data-In.**