Use tag icons + colors in forms and views

Summary:
Better tags selector in search and post forms
- Better tags selector using select2 for the tags search field
- Similar use of select2 for create/edit post forms too
- Display tag icon and color on post views
- API: Don't return "#" for empty tag color

Test Plan:
- Load homepage, click in tag search field.
See dropdown of tags with icons and color
- Click create post button, select a form. See similar tags selector here
- Edit a post - similar tags selector here.
- See tag icons on post listing and in post detail view

Reviewers: seth, eyedol, shadowhand

Reviewed By: seth, eyedol, shadowhand

Differential Revision: https://phabricator.ushahidi.com/D160