Use AuthManager on special pages

Rewrite authentication-related special pages to use AuthManager.
All the changes mentioned below only take effect when
$wgDisableAuthManager is false.

LoginForm is rewritten to use HTMLForm and split into UserLogin
and CreateAccount; ChangePassword and PasswordReset are rewritten;
ChangeEmail and Preferences are updated. Four new special pages
are added to handle the new capabilities of AuthManager (linked
accounts, secondary authentication providers): LinkAccounts,
UnlinkAccounts, ChangeCredentials, RemoveCredentials.

The old form-based hooks (ChangePasswordForm, UserCreateForm,
UserLoginForm) are deprecated. A new, more generic hook is
available to alter the forms (AuthChangeFormFields);
form changes that involve new fields should be done via
$wgAuthManagerConfig.

UserLoginComplete is limited to web-based login; for more
generic functionality UserLoggedIn can be used instead.

Hooks that assume password-based login (PrefsPasswordAudit,
AbortChangePassword) are removed; the first functionality
is replaced by ChangeAuthenticationDataAudit, the second is
handled by AuthManager. LoginPasswordResetMessage is removed,
the functionality can be recreated via authentication providers.

There are several smaller backwards incompatible changes:

• Adding fields to the login/signup forms by manipulating the template via the extraInput/extrafields parameters is not supported anymore. Depending on the authn configuration the login/signup process might be multistep and it would be complicated to ensure that extensions can access the data at the right moment. Instead, you can create an AuthenticationProvider which can define its own fields and process them when the authentication is over. (There is B/C support for a transitional period that works with the default login form, but might break with configurations that require multiple steps or redirects.)
• Removed cookie redirect check. This was added in 2003 in 9ead07fe9 for the benefit of bots, but with MediaWiki having an API these days there is little reason to keep it. Same for the wpSkipCookieCheck flag (added in 2008 in 29c73e8265).
• Instead of embedding a password field on sensitive special pages such as ChangeEmail, such pages rely on AuthManager for elevated security (which typically involves requiring the user to log in again unless their last login was more than a few minutes ago). Accordingly, wgRequirePasswordforEmailChange is removed.
• Special:ChangePassword requires login now.
• Special:ResetPassword now sends a separate email to each user when called with a shared email address.
• the Reason field had a message with 'prefsectiontip' class which was sorta broken but used in extensions for formatting. HTMLForm does not support that, so this commit turns it into a help message which will break formatting. See https://gerrit.wikimedia.org/r/#/c/231884

Bug: T110277
Change-Id: I8b52ec8ddf494f23941807638f149f15b5e46b0c
Depends-On: If4e0dfb6ee6674f0dace80a01850e2d0cbbdb47a