Add stats API endpoint, refs T98

Summary:
Adding Visualization tab for inital data viz
Created a stats api for easy pulling of stats, currently handling count of tags, posts and users.

Test Plan: bin/behat --config application/tests/behat.yml application/tests/features/api.stats.feature --format progress

Reviewers: eyedol, dkobia, rjmackay

Reviewed By: rjmackay

Subscribers: srutto, auremoser

Maniphest Tasks: T98

Differential Revision: https://phabricator.ushahidi.com/D157