Add a way to define an application password

[dd32]

As 2FA can't be used in all systems (for example, HTTP Basic Authentication) we need a way for users to be able to generate an application password for use with those systems.

This should be a password that is generated by us, is long, contains no special characters that would cause issues with escaping, and be non-memorable / non-guessable.

This password should be able to be re-generated at will, at which time, the other instances of the password become invalid.

For an example of what such a password could look like, with 64 [A-Za-z0-9] characters.

KdkcGr8yB6ZqUjCNJXVE6x4ujNgviERd8HLyvVoYyPam2NKE2gJGoGE8x2ZcPLZw

[dd32]

This would effectively implement https://meta.trac.wordpress.org/ticket/5555