Unify action checks

Fixes: https://github.com/thoughtbot/administrate/issues/1861

When referring to a route in the code, we run two checks:

• valid_action? is true if the route is defined, false otherwise.
• show_action? is expected to be overridden by developers in order to implement authorization. For example, it's implemented by Administrate::Punditize in order to integrate Administrate with Pundit. It should return true if the current user can access a given route, false otherwise.

These two check should (almost) always happen together. For this reason, our code is peppered with if statements where both are checked... and a few others where we forget one or the other.

These checks should be unified into a single method call, in order to avoid issues like the one described at https://github.com/thoughtbot/administrate/issues/1861. Here I propose such a method, which I call accessible_action?.

The original methods should still exist, as they do have their uses individually. The new method will delegate to the existing ones.

I have also renamed the two existing methods to something that I hope will make their intent clear:

• valid_action? becomes existing_action?
• show_action? becomes authorized_action?
• In order to provide a clear upgrade path, the old names still exist and work, but they show a deprecation warning when used. They can be removed properly at a later version of Administrate.

Open question!

The original methods have a signature the_old_question?(action, resource). I have flipped the order of parameters to be the_new_question?(resource, action), as I find that more natural. But if people think the original order is better (or have some other proposal), I'm happy to oblige.

Also! The old valid_action? has a default parameter value (the full signature is valid_action?(name, resource = resource_class)). The new version doesn't have it, as I can't see that it has a good use case. Incidentally, this makes sense with the parameter order flip just described.