House cleaning. - #8

Hi there! 👋

I had some time, so I went through the current code base (i.e., the auth-code-flow branch, which this PR thus targets), with an army of house elves, and fixed, updated and improved a few things here and there.

Sorry for not splitting up commits. I did this on purpose, though, because otherwise we would've ended up with 42 (!) commits, ... or so.
I did not do any real changes to functionality, though, but see for yourself...

What Does This Pull Request Include?

• General:
  • Replace almost all mentions of OAuth1 with OAuth2.
  • Require WordPress version 4.8 or higher.
  • Bump tested WordPress version to 4.8.
• Code:
  • Remove unused stuff (e.g., variables, namespace imports).
  • Add/Adapt phpDoc blocks for methods with either parameters or return values, or both.
  • Add missing namespace imports.
  • Add a few type hints.
  • Make WP\OAuth2\Client::check_redirect_uri() return true or false.
  • Improve and streamline WP\OAuth2\Endpoints\Authorization::handle_request().
  • Ensure authorization code expiration is numeric, and return an int.
  • Make WP\OAuth2\Tokens\Token::is_valid() abstract, and not static anymore.
  • Introduce abstract WP\OAuth2\Tokens\Token::get_meta_prefix() method.
  • Make PhpStorm (or any other decent IDE) aware of $client in the login template.

What's Left?

• Coding Standards:
  • Every third file looks different than the others.
  • Where to put blank lines (both in code and comments)?
  • Import classes in the global namespace, yes or no?
• admin.php:
  • I did not touch this file at all.
  • It is broken, though.
  • The referenced file lib/class-wp-rest-oauth1-admin.php does not exist in this plugin.
  • oauth1 all over.
• inc/class-scopes.php:
  • I did not touch this file at all.
  • I don't get it. 😅
  • It's not referenced/used at all.
  • Properties are either defined and not used, or not defined, but set. 🤔
• inc/endpoints/class-authorization.php:
  • The handle_request() method uses OAuth2\get_grant_types(), which returns the unvalidated result of a filter, oauth2.grant_types.
  • There is direct, but unchecked access to assumed methods.
  • In my opinion, the OAuth2\get_grant_types() function should both validate the data (and remove invalid handlers) and state WP\OAuth2\Types\Type[] instead of array as return type.
• inc/tokens/class-authorization-code.php:
  • I assume the $args parameter in WP\OAuth2\Tokens\Authorization_Code::validate() is for inheritance...?
• Possibly other stuff...

Again, this PR targets the auth-code-flow branch. If you want me to make it target master instead, just tell me.

Cheers,
Thorsten