Remove deprecated '#prebuild/' context URL prefix

[svenefftinge]

When we have projects, we should no longer allow anyone to trigger prebuilds on arbitrary repos. Instead, the context URLs should become an admin-only feature.

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Edits by @gitpod-io/engineering-webapp:

• Prebuilds should only be triggered via the Run Prebuild button in the dashboard or the Gitpod API.
• If Admins need to trigger prebuilds, we can also add a Run Prebuild button to the Admin dashboard.

However, before the prefix can be definitely removed, we need to ensure that we're not removing functionality or breaking user workflows without adequate replacement.

Depends on

• Support adding Projects & enabling Prebuilds for Self-Managed GitLab and Bitbucket: [projects] enable webhook registration for bitbucket.org and self-managed gitlab #7422
• Make sure Prebuilds can actually be enabled for Self-Managed GitLab: Cannot add self-hosted Gitlab repos as a Gitpod Project #7781
• Remove the also-deprecated and never-announced #incremental-prebuild/ URL prefix

[shaal]

I have 2 questions regarding this issue:

1. Would it be possible to add to this issue, that only a person with permission to trigger a prebuild, will be able to see the output of the prebuild? (related to running tmate during dockerfile build https://github.com/shaal/DrupalPod/blob/debug-with-tmate/.gitpod/.gitpod.Dockerfile#L29-L34)
2. The other URL params (that creates environment variables in the workspace) will remain available for everyone to use, is that correct?

[jankeromnes]

Given that this feature is broken (see #6688 (comment)) I don't think it makes sense to spend a lot of effort to fix it just for admins.

Instead, it's probably better to make it redirect to the most sensible place (e.g. the Prebuilds page of the Project, if you have access to it, so that you can click on the new Run Prebuild button).

[jankeromnes]

Now that #7422 is merged and deployed, I think we no longer need the manual prebuild prefix.

@jldec maybe we can rename this issue to "Deprecate and remove Prebuild Context URLs" and schedule it?

It's a quick fix, and I believe it's both no longer useful and actually broken and harmful from a security standpoint (since anyone can trigger Prebuilds for any context URL).

[jankeromnes]

Side note: the /#incremental-prebuild/ prefix should go away too. It's equally broken (no project association) and was never announced in the first place (it was more of an internal hook for us to test Incremental Prebuilds).