Changes for ember compatibility

[begedin]

What was done

• Changed the error format for validation errors to conform JSON API specs and guidelines. Ember expect that specific format and handles it automatically, so the alternative of customising error handling client-side would take longer.
• Made it so, when creating a post, the controller reloads the record before responding to the request, reason being, the post number is being added at the database level, so we need to reload to get that information back.
• CommentController#index was lacking the "filter by ID functionality", so I added it and specs to support the feature.
• PostController.index was missing filtering by status, so I added that as well
• Made it so slug only regenerates if the source value it's generated from changes. Simplifies some things to a degree.
• OrganizationMembershipControler#index was filtering by member_id instead of id. Fixed that bug.
• CategoryController#index was missing some preloads
• User was missing the default "signed_up" state upon creation, so the onboarding process would never trigger.
• Some authorization policies needed a changeset to properly determine user permissions, so I added custom authorization functionality, which accepts a changeset, namely, policies for UserCategory, UserRole and UserSkill

[npendery]

Should these two methods be added to controller helpers since they are used in multiple controllers?

[begedin]

I'm not really sure. Maybe a brand new authorization_helpers would make more sense.

[npendery]

Looks good! Just had one comment. Will look over again though later to make sure I didnt miss anything

[begedin]

@npendery I've moved the two functions into CodeCorps.AuthenticationHelpers. It makes the most sense, really, since we already have related functions there.