Prebuild trigger checks

[filiptronicek]

Description

This change introduces the following functionality around activity-based prebuilds:

• Adds a warning for repositories that have received a webhook event for the most recent commit about considering an update to activity-based prebuilds
• Automatically changes prebuild trigger strategy from activity-based to webhook-based when we receive a webhook event
• Automatically changes prebuild trigger strategy from webhook-based to activity-based when creating a workspace and the repository not sending a webhook push event for the last commit

Related Issue(s)

Fixes ENT-601

How to test

https://ft-webhook870fa6143a.preview.gitpod-dev.com/workspaces

1. Import a repo
2. Enable prebuilds on it
3. See warning
4. Remove the webhook and push a new commit
5. Observe that the warning disappears

[geropl]

@filiptronicek It would be great to align on the goals in sync. I feel I'm a bit lost on the "make webhook the new/old" default, but might also be mis-reading things. 👍

[geropl]

Code LGTM, tested and works! ✔️

(please revert the debug commit before merge 😇 )