Merge torrust#588: Udate Index TOML config file

fc19076 fix: [torrust#587] update Index TOML config file (Jose Celano)

Pull request description:

  Relates to: torrust/torrust-index#670

  These two config option in the Index TOML config file have been removed:

  ```toml
  [auth]
  email_on_signup = "optional"

  [mail]
  email_verification_enabled = false
  ```

  We need to replace them with:

  ```toml
  [registration]
  [registration.email]
  ```

ACKs for top commit:
  josecelano:
    ACK fc19076

Tree-SHA512: d2f02d04c2d08ce3ba29ee5208d720d45db7b1dd376a5f31dc6b22fb6396c7ea9f34577072f56678a9bd77f9d2f75d3989842f7395af49c57c9b26438dca3f12

share/default/config/index.private.e2e.container.sqlite3.toml

@@ -11,3 +11,9 @@ connect_url = "sqlite:///var/lib/torrust/index/database/e2e_testing_sqlite3.db?m
 [mail.smtp]
 port = 1025
 server = "mailcatcher"
+
+[registration]
+
+[registration.email]
+#required = false
+#verified = false

share/default/config/index.public.e2e.container.sqlite3.toml

@@ -11,3 +11,9 @@ connect_url = "sqlite:///var/lib/torrust/index/database/e2e_testing_sqlite3.db?m
 [mail]
 port = 1025
 server = "mailcatcher"
+
+[registration]
+
+[registration.email]
+#required = false
+#verified = false