Better handling for revoked credentials on the admin page.
Better handling of GitHub OAuth errors when sending digest emails.

Fail silently if we can't even get an email address, otherwise send an error
email to the user.
More aggressively persis the user's email address in the account object.
Add basic robots.txt file.
Don't retry vintage calculation if the repo can't be loaded.

The user has most likely deleted their account.
Fix private repositories checkbox not being visible in Firefox.

Firefox doesn't support generated content in before/after pseudo classes applied
to <input type="checkbox"> elements. Switch instead to adding the custom
checkbox glyph to the <label> for the checkbox.

Thanks to @snarfed for reporting this.