Move alertify calls to a reusable module

Summary:
- Create notify module
- Replace alertify calls in posts/tags/users lists
- Replace alertify calls in create/edit post modals

Test Plan:
Go to posts, tags or users list.
Try to delete items,
Try to publish/unpublish posts.
Test bulk actions too.
Edit a post, save the post - check notifications are as expected.

Reviewers: aMoniker, shadowhand

Reviewed By: shadowhand

Differential Revision: https://phabricator.ushahidi.com/D413