Replace silverstripe/react-injector with admin injector

[emteknetnz]

Follow on from silverstripe/silverstripe-mfa#584 (comment)

https://github.com/silverstripe/react-injector/ is an unmaintained repository that is something of an out of date clone of lib/Injector that is in silverstripe/admin. silverstripe/react-injector is only used by silverstripe/mfa to get the injector functionality without being logged in to admin. It does not appear to be used by silverstripe/silverstripe-login-forms.

Injector should instead be exported as a separate bundle by silverstripe/admin so that it can be consumed both inside and outside of admin.

Acceptance criteria

• Injector is exported as its own bundle in silverstripe/admin
• All existing admin functionality that relies on injector continues to work
• silverstripe/mfa is updated to use the exported bundle rather than silverstripe/react-injector
• silverstripe/react-injector is archived

[emteknetnz]

I'm questioning whether this is even worth doing since we intend to replace injector entirely at some point

While this existing code duplication obviously isn't great, it's also not broken. Seems like we're better off just leaving this as is.