:honeybee: engineering: enable explorers on staging servers

[Marigold]

Enable editing of explorers on staging servers.

• New staging servers create a branch in owid-content
• Explorer steps in ETL commits to that branch on the staging server (previously all changes were uncommitted) (e.g. etlr explorers/minerals --export)
• Editing Explorer TSV from Admin commits to the branch
• Merging the branch pushes the branch to owid-content repo. It doesn't automatically merge it with master, but one can create a PR from it and merge manually