Put the language redirect subscriber behind its own scope and setting

Problem/Motivation

MyrestRedirectEventSubscriber::onRedirectToDefaultLanguage() rewrites redirects that carry a language prefix, which was written for the node add forms of one site. It runs for every response of the site that is a redirect, so on any other site it changes the behaviour of forms and workflows this module knows nothing about. It is the third of the three redirect behaviours; the other two are already behind settings.

Proposed resolution

• Add redirect_admin_forms_to_default_language to the same "Redirects for a headless back end" section, default off.
• Scope it to the routes it was written for rather than to every redirect response, and guard it with isMainRequest() like its siblings.
• Update hook that switches it on for existing sites, the same way it was done for the node redirect.
• Functional coverage of both states.

Remaining tasks

Everything.