Rename to "Private Mirrors App"

[ahpook]

Is your feature request related to a problem?

As part of moving to a hosted app and wider beta, we want to rename ICF to "Private Mirrors App". This meets the design goals of concision, disambiguation vs a built-in GitHub feature, and consistency with its actual functionality.

Describe the solution you'd like

Usage guidance:

• whenever it's fully written out for the first time in text it should be called "Private Mirrors App"
• after the first usage it can be abbreviated "PMA" (s/ICF/PMA/g)
• inside of the product, text strings can use the terminology "Private Mirrors" to refer to the app itself (e.g. page title elements, headers)
• if it's clear from context, docs can use "the app" or "PMA" interchangably
• the repo can be named 'private-mirrors'
• the hostname can be 'private-mirrors.githubapp.com'
• internal code references can use "private mirrors" in snake/camel/whatever case since the "app" part is obvious

Describe alternatives you've considered

No response

Additional context

No response

[sutterj]

@ahpook Did you start working on this?