Move the limit check before triggering a prebuild

[gtsiolis]

Problem to solve

Currently, when you try to open a private repository and you don't have permissions to use private repositories with Gitpod (see Pricing) you get a Limit Reached modal which is expected. However, when you try to trigger a prebuild using the URL prefix, Gitpod goes through the first steps required for triggering the prebuild before reaching the modal.

Modal	Prebuild

Proposal

Although the plan is to make prebuild context URLs admin only (see #4353) it could be worth considering moving the limit check for this before triggering a prebuild. 🤷

Feel free to close this if needed. 📕

TBD (To be discussed)

[stale]

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