UX: Add option to use fullpages for login and signup (#29034)

This adds dedicated routes for /login and /signup, replacing the use of modals. Currently, this is behind the experimental_full_page_login feature flag. It also includes some small consistency fixes related to formatting, spacing, icons, and the loading of certain elements

config/locales/client.en.yml

@@ -2337,6 +2337,7 @@ en:
       associate: "Already have an account? <a href='%{associate_link}'>Log In</a> to link your %{provider} account."
       activation_title: "Activate your account"
       already_have_account: "Already have an account?"
+      no_account_yet: "Don't have an account?"
       progress_bar:
         signup: "Sign Up"
         activate: "Activate"
@@ -2378,7 +2379,7 @@ en:
     login:
       header_title: "Welcome back"
       subheader_title: "Log in to your account"
-      title: "Log in"
+      title: "Log In"
       username: "User"
       password: "Password"
       show_password: "Show"

config/locales/server.en.yml

@@ -1871,6 +1871,7 @@ en:
     pending_users_reminder_delay_minutes: "Notify moderators if new users have been waiting for approval for longer than this many minutes. Set to -1 to disable notifications."
     persistent_sessions: "Users will remain logged in when the web browser is closed"
     maximum_session_age: "User will remain logged in for n hours since last visit"
+    experimental_full_page_login: "EXPERIMENTAL: Replace the login/signup modal with a full page login/signup form."
     ga_version: "Version of Google Universal Analytics to use: v3 (analytics.js), v4 (gtag)"
     ga_universal_tracking_code: "Google Universal Analytics tracking code ID, eg: UA-12345678-9; see <a href='https://google.com/analytics' target='_blank'>https://google.com/analytics</a>"
     ga_universal_domain_name: "Google Universal Analytics domain name, eg: mysite.com; see <a href='https://google.com/analytics' target='_blank'>https://google.com/analytics</a>"

config/site_settings.yml

@@ -619,6 +619,11 @@ login:
     default: 1440
     min: 1
     max: 175200
+  experimental_full_page_login:
+    default: false
+    client: true
+    hidden: true
+
 users:
   min_username_length:
     client: true