Land the frontend-base branch

[arbrandes]

Description

This repository's frontend-base branch has carried its migration since inception, taking ports of the features and bugfixes that landed on master along the way. It is now what the repository should be, and master is what it replaces.

Landing it is not a reconciliation. The two branches have diverged too far for an ordinary merge, and there is nothing left to reconcile because the ports already happened. The migration branch takes over instead, joining both histories while keeping its own content as the result. openedx/frontend-base#243 has the procedure.

The migration PR is #906. Landing it means taking it out of draft, getting it reviewed, and merging it.

End state

Once landed, master is the App Repository template, and the legacy standalone MFE template goes on living on legacy-mfe. Unlike the app repositories in this epic, this one is no longer part of a named Open edX release, so its openedx.org/release annotation stays null and there is nothing for legacy-mfe to be cut for. It is also not on the list in openedx/public-engineering#563, so there is no stable line to rename here.

Checklist

• Take feat!: migrate to frontend-base #906 out of draft, get it reviewed, and merge it
• Cut and push legacy-mfe from master's tip
• Merge frontend-base into master with -s ours, then fast-forward master onto it
• Delete the frontend-base branch
• Confirm openedx.org/release in catalog-info.yaml stays null
• Repoint the README's legacy-template link from master to legacy-mfe, and drop the warning banner now that the default branch is the frontend-base template
• Point branch protection at the new layout