Display "prebuild trigger" in the UI

Currently users cannot distinguish at-all/easily between:

• no webhook received at all (installation misconfigured)
• webhook received but not run due to global rate-limiter
• webhook received but not run because of user config
• what caused a particular prebuild to be executed

This issue is the first one in a series that seeks to enable the user to understand just that. To that end, we need to keep track of "prebuild triggers".

A simple 🛹 for displaying such information would be in a table-like fashion, similar to the "Branches" tab on a project (which it might replace, eventually). Each row would contain:

• the timestamp (ordered DESC)
• what triggered it (commit on repo&branch)
• state (+ message): possibly showing a link to the prebuild details page if there is one

This can easily be expanded upon (show prebuild status if present, filtering by branch, etc.), but that's not part of the first iteration.

Start by locating the UI for the project's "Branches" tab and the prebuild details page, then trace how prebuild triggers are recorded. The first iteration is done when a table lists triggers newest first with timestamp, repository branch and commit, state and message, plus a prebuild details link when available.