Avoid accumulating large amounts of webhook event data in d_b_webhook_event

[jldec]

Followup to PR #11356

The JSON for raw events in d_b_webhook_event is quite large - and events will accumulate fast. Let's reduce the data which is retained long term, and avoid storing information which is never relevant to prebuilds.

[AlexTugarev]

(node: 29) UnhandledPromiseRejectionWarning: QueryFailedError: ER_DATA_TOO_LONG: Data too long for column 'rawEvent' at row 1

@jldec, can we schedule this?

[jldec]

scheduled (no need to ask :))

[jldec]

First step here is probably to reduce the size of data stored by removing large unnecessary json like git changes