Add SelectableList behaviors

Summary:
Remove repeated logic for selecting models within lists
and move it to a reusable behavir

Test Plan:
Go to posts, tags or users lists and confirm selecting works
- Select/unselect items
- Trigger a bulk action
- Check select-all button works

Reviewers: aMoniker, shadowhand

Reviewed By: shadowhand

Differential Revision: https://phabricator.ushahidi.com/D419