Fix 2FA-based login for LDAP users

The OTP input form is shared by both LDAP and standard logins, but when
coming from an LDAP-based form, the form parameters aren't nested in a
Hash based on the `resource_name` value.

Now we check for a nested `remember_me` parameter and use that if it
exists, or fall back to the non-nested parameters if it doesn't.

Somewhat confusingly, the OTP input form _does_ nest parameters under
the `resource_name`, regardless of what type of login we're coming from,
so that allows everything else to work as normal.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/18185

See merge request !4493

Allow custom text on 'almost there' page

Add a new application setting, after_sign_up_text. This is text to be
rendered as Markdown and shown on the 'almost there' page after a user
signs up, but before they've confirmed their account.

Closes #14098.

See merge request !4098

Added minimum password length to password field

Add a placeholder to the password field in the register form with the minimum number of characters required. Also added a pattern to the field to stop it submitting if less than that.

Closes #17765

See merge request !4458

Add a new application setting, after_sign_up_text. This is text to be
rendered as Markdown and shown on the 'almost there' page after a user
signs up, but before they've confirmed their account.

Tweak the styles for that page so that centered lists look reasonable.

The OTP input form is shared by both LDAP and standard logins, but when
coming from an LDAP-based form, the form parameters aren't nested in a
Hash based on the `resource_name` value.

Now we check for a nested `remember_me` parameter and use that if it
exists, or fall back to the non-nested parameters if it doesn't.

Somewhat confusingly, the OTP input form _does_ nest parameters under
the `resource_name`, regardless of what type of login we're coming from,
so that allows everything else to work as normal.

- Move the `authenticate_with_two_factor` method from
  `ApplicationController` to the `AuthenticatesWithTwoFactor` module,
  where it should be.

Closes #17765