Add `isBlank`

[kidonng]

There are several situations where the repo may only show a takedown notice:

• "This repository has been disabled." (TOS takedown)
• "Repository unavailable due to DMCA takedown." (DMCA takedown)
• "Repository unavailable in your location" (GOV takedown)

Additionally, an issue may be deleted and only the repository header is shown.

I propose to add a isBlank detection for them. There are several ways to check for them:

• document.title !== 'GitHub · Where software is built': this is already used in isIssue
• Check for existence of main .blankslate: this may catch more cases and if GitHub ever changes their tagline again 😄

[kidonng]

Turns out .blankslate is already used in multiple places of Refined GitHub.

[fregante]

Sounds good