…rther extension to non-json outputs too.

Author: Hari Shreedharan <hshreedharan@apache.org>

Closes #6273 from harishreedharan/json-to-api and squashes the following commits:

e14b73b [Hari Shreedharan] Rename `getJsonServlet` to `getServletHandler` i
42f8acb [Hari Shreedharan] Import order fixes.
2ef852f [Hari Shreedharan] [SPARK-7750][WebUI] Rename endpoints from `json` to `api` to allow further extension to non-json outputs too.