Make the 'Run Prebuild' button more consistent

[jankeromnes]

Is your feature request related to a problem? Please describe

Currently, Gitpod only accepts to run a prebuild for a commit that:

• Belongs to a repository for which a Gitpod Project exists
• Does not already have a successful prebuild associated

The problem is with the definition of "successful" -- Gitpod currently allows re-running prebuilds that failed due to system reasons (e.g. timed out), but not prebuilds that finished "successfully" but their init task finished with a non-zero error code (shown as "failed" in the UI, but not re-runnable, which is confusing).

Also, in general, making the 'Run Prebuild' action conditional makes the UX confusing: The 'Run Prebuild' action is seen as inconsistent and unreliable (sometimes it does something, sometimes it refuses).

Describe the behaviour you'd like

To fix this confusing situation, we could:

• Always offer the 'Run Prebuild' / 'Re-run Prebuild' action, regardless of what state other prebuild are or aren't in

Describe alternatives you've considered

Additional context

There might be a few side effects to allowing multiple (potentially "successful") prebuilds for a single commit. However, we're pretty sure that we can handle those separately, and most of the system already supports multiple prebuilds for a single commit (e.g. multiple failed ones and one "successful" one).