Fix broken post pagination by adding "total_count" back, refs T438

Summary:
D216 lost the `total_count` being added to the post collection response.

Add it back in by capturing the search total in the repo and exposing a new
method, `getSearchTotal`, to retrieve it.

Test Plan: View the home page.

Reviewers: aMoniker, rjmackay

Reviewed By: rjmackay

Subscribers: eyedol

Maniphest Tasks: T673, T438

Differential Revision: https://phabricator.ushahidi.com/D329