Only ramp post types that are supported by Gutenberg

[pyronaur]

This fixes #13 by preventing ramp_for_gutenberg_load_gutenberg from enabling Ramp on post types that can't support Gutenberg (related Gutenberg issue: WordPress/gutenberg#3066 )

Only post_type argument is affected by this PR
Single post_ids can still be enabled in Ramp at the moment, even if they're not supported by Gutenberg. I'm a bit cautious and unsure whether we should check those too, because Ramp will have to run get_post_type( $post_id ) for each post in the post_ids list on every admin page load.

Test plan:

1. Install a plugin with a custom post type that doesn't want an editor UI. For example: "WP RSS Aggregator"
2. Make sure WP_DEBUG is enabled
3. Use the following code to enable ramp:

ramp_for_gutenberg_load_gutenberg( [ 'post_types' => [ 'post', 'wprss_feed' ] ] );

4. Experience an error message that looks something like this:

Notice: ramp_for_gutenberg_load_gutenberg was called <strong>incorrectly</strong>. 
Cannot enable Gutenberg support for post type "wprss_feed" Please see <a href="https://codex.wordpress.org/Debugging_in_WordPress">Debugging in WordPress</a>

[mattoperry]

Looks good just one question inline.

[mattoperry]

@justnorris wondering if the validation should happen here rather than before the option is written -- ie: would we want to assign invalid criteria to self::$criteria ?

[pyronaur]

Ok, I made some changes to how this works.

When storing criteria (even if just temporarily) - sanitize the array first, just like we did before. But instead of directly updating it in options, keep the data in a private class variable.

Then, on admin_init - validate the options to values that we expect and if they pass - store them.

Thank you @mattoperry for the review! I think this is a lot better.