REST API: Modify plugins endpoints path.

Instead of
`/sites/:site_id:/plugins/:urlencoded_plugin_filename_without_dot_php:/<
action>/`, use : `/sites/:site_id:/plugins/<action>/ POST { plugin_id:
plugin_filename }`.

Adds simplicity to the path and flexibility for future bulk actions.
REST API: Fix endpoints path comment.
REST API: Modules: Stricter response format.

Adds more control over the information returned about each module.
REST API: Fix example request data.
REST API: Remove code tag ¿¿¿???
REST API: Modules endpoint refactoring.

Move shared code to the parent class.
TEST: Jetpack Endpoints: Fix failing test.

Caused by the refactoring of plugins methods in
f23882b64f29db77014fbfbc68cd99eb950c72ad