Finishing the install requires creating a GitHub OAuth app #654

Problem

The last step of the install wizard asks for a GitHub OAuth app's client ID and secret and then sends the user to GitHub. Until that succeeds there is no admin and the site cannot be managed. For a new user this is the hardest part of the install:

• They have to know what an OAuth app is, create one, and get the callback URL right. The wizard page is the only guidance.
• It cannot be done for a site that is not reachable at the callback URL they registered.
• There is no other way to create the first admin. Email login exists, but only for visitors and only once SMTP is configured.

It is also the one step the install smoke test (#651, #652) cannot cover, because CI cannot authorise a GitHub app.

Ideas

• Step-by-step instructions on the wizard's auth page, with the exact callback URL for this site filled in and a link to GitHub's "new OAuth app" form.
• A way to create the first admin without GitHub, such as a one-time setup code printed to the server log, or a password for the admin account.

Found while scoping #651.