REST API: Improve endpoint paths.

POST /sites/%s/plugins/%p { active: false } (deactivates module p on
site s)
POST /sites/%s/modules/%m { active: true } (activates module m on site
s)
REST API: Tidying spaces.
REST API: Use safehtml for module's description
REST API: Add get module endpoint.
REST API: Define common properties in the parent.
REST API: Improve error identification.
REST API: Not found plugin should return 404.
REST API: Ooops, wrong parent class.
REST API: Add request format.
Merge pull request #899 from Automattic/proper-endpoints-paths

Proper endpoints paths