Proper endpoints paths

Adds:

GET /sites/%s/plugins/%p  (get plugin p info on
site s)
GET /sites/%s/jetpack/modules/%m (get module m info on site
s)

Fixes:

POST /sites/%s/plugins/%p { active: false } (deactivates plugin p on
site s)
POST /sites/%s/jetpack/modules/%m { active: true } (activates module m on site
s)