CMS API: Drop support for XML requests

[thalesmiguel]

What this PR does / why we need it:

This PR removes support for XML format requests from all CMS endpoints. It also makes JSON the standard request format for the CMS API.

Which issue(s) this PR fixes

THREESCALE-9433

Verification steps

The following endpoints should only respond to JSON requests. Any other format should return a 406 status.

Verb	Endpoint	Controller#Action
GET	/admin/api/cms/sections/:section_id/files	admin/api/cms/files#index
GET	/admin/api/cms/sections	admin/api/cms/sections#index
POST	/admin/api/cms/sections	admin/api/cms/sections#create
GET	/admin/api/cms/sections/new	admin/api/cms/sections#new
GET	/admin/api/cms/sections/:id/edit	admin/api/cms/sections#edit
GET	/admin/api/cms/sections/:id	admin/api/cms/sections#show
PATCH	/admin/api/cms/sections/:id	admin/api/cms/sections#update
PUT	/admin/api/cms/sections/:id	admin/api/cms/sections#update
DELETE	/admin/api/cms/sections/:id	admin/api/cms/sections#destroy
GET	/admin/api/cms/files	admin/api/cms/files#index
POST	/admin/api/cms/files	admin/api/cms/files#create
GET	/admin/api/cms/files/new	admin/api/cms/files#new
GET	/admin/api/cms/files/:id/edit	admin/api/cms/files#edit
GET	/admin/api/cms/files/:id	admin/api/cms/files#show
PATCH	/admin/api/cms/files/:id	admin/api/cms/files#update
PUT	/admin/api/cms/files/:id	admin/api/cms/files#update
DELETE	/admin/api/cms/files/:id	admin/api/cms/files#destroy
PUT	/admin/api/cms/templates/:id/publish	admin/api/cms/templates#publish
GET	/admin/api/cms/templates	admin/api/cms/templates#index
POST	/admin/api/cms/templates	admin/api/cms/templates#create
GET	/admin/api/cms/templates/:id	admin/api/cms/templates#show
PATCH	/admin/api/cms/templates/:id	admin/api/cms/templates#update
PUT	/admin/api/cms/templates/:id	admin/api/cms/templates#update
DELETE	/admin/api/cms/templates/:id	admin/api/cms/templates#destroy

Special notes for your reviewer:

[jlledom]

There are some endpoints in your list that don't exist AFAIK:

Verb	Path	Action
GET	/admin/api/cms/sections/new	admin/api/cms/sections#new
GET	/admin/api/cms/sections/:id/edit	admin/api/cms/sections#edit
GET	/admin/api/cms/files/new	admin/api/cms/files#new
GET	/admin/api/cms/files/:id/edit	admin/api/cms/files#edit

Are those routes valid and working? because there's no action in the controllers for them.

Also, I didn't even know this one existed:

Verb	Path	Action
GET	/admin/api/cms/sections/:section_id/files	admin/api/cms/files#index

I think we should remove it, if we provide sections/:section_id/files we should also provide sections/:section_id/sections and sections/:section_id/templates. And we have an issue (THREESCALE-9191) to filter results by any parameter, so the client will be able to retrieve the same results by filtering by section_id.

[thalesmiguel]

There are some endpoints in your list that don't exist AFAIK:

Yep... Those are "new" and "edit" endpoints that come from the "resources" Rails routes helper. Those don't really make sense for an API.

Also, I didn't even know this one existed:
I think we should remove it, if we provide sections/:section_id/files we should also provide sections/:section_id/sections and sections/:section_id/templates. And we have an issue (THREESCALE-9191) to filter results by any parameter, so the client will be able to retrieve the same results by filtering by section_id.

Agreed!

Since those changes go out of the scope of the PR, I opened #3266 to remove the endpoints you mentioned @jlledom ✌️