Temporary work around for multiple deployments of Pages

We need to unblock merge requests caused by problems with multiple deployments of Pages. See this internal discussion.

The plan is:

1. Open MRs to only run the pages job on the main branch. This should prevent MR pipelines from clobbing the Pages site.
2. Disable Use multiple deployments in Deploys > Pages.
3. Wait for gitlab!151555 (merged) to hit production.
4. Undo step 2.
5. Undo step 1.