Enable prebuilds for all branches when adding a project

[gtsiolis]

Problem to solve

For GitHub repositories, when installing the GitHub App, prebuilds are enabled by default for the default branch. For enabling prebuilds for all branches, a user will need to defien this in .gitpod.yml, see relevant docs.

For GitLab and Bitbucket repositories, when adding aproject a webhook is installed. However, enabled are not enabled, see #7424 and pending PR in #7422. Prebuilds are also not enabled when adding a project, following the pattern used for GitHub repositories.

This makes enabling prebuilds for all branches a less discoverable option and more difficult to set up.

Proposal

To make prebuilds more useful, we could consider 🅰️ enabling prebuilds for all branches by default or 🅱️ introduce a preference in project settings that allows enabling or disabling prebuilds for branches, pull requests, etc.

In the spirit of MVC (minimum viable change), enabling prebuilds for all branches could a good MVC step. ➿

[jldec]

Closing as non-reproducible. Please file a new issue with repro steps if you believe this is still a problem. Thanks.