Let the platform login control user login/register flow

[CodeWithEmad]

Forcing users to go to Authn MFE results in a bad UX. We need to let the platform decide on login and registration.
Imagine this scenario: you have a TPA setup and want users to log in there directly. you can have this by adding "THIRD_PARTY_AUTH_HINT": "some-provider-slug" and if users go to /login it redirects them to the TPA login/registration page but the current logic takes users to the Authn MFE and they need to know that they have to click on another button which results in a bad UX!

edx-platform/lms/templates/header/navbar-not-authenticated.html

Lines 51 to 69 in 3c571f3

	% if should_redirect_to_authn_mfe:
	<div class="mobile-nav-item hidden-mobile nav-item">
	<a class="register-btn btn" href="${settings.AUTHN_MICROFRONTEND_URL}/register${login_query()}">${_("Register for free")}</a>
	</div>
	% else:
	<div class="mobile-nav-item hidden-mobile nav-item">
	<a class="register-btn btn" href="/register${login_query()}">${_("Register for free")}</a>
	</div>
	% endif
	% endif
	% if should_redirect_to_authn_mfe:
	<div class="mobile-nav-item hidden-mobile nav-item">
	<a class="sign-in-btn btn" href="${settings.AUTHN_MICROFRONTEND_URL}/login${login_query()}">${_("Sign in")}</a>
	</div>
	% else:
	<div class="mobile-nav-item hidden-mobile nav-item">
	<a class="sign-in-btn btn" href="/login${login_query()}">${_("Sign in")}</a>
	</div>
	% endif