Feature/fs 1343 add image tag

[staturecrane]

Adds FS-1343, the Filestack image tag.

Use is similar to previous version, but now with access to all Filestack transformations (except those which do not return an image).

<%= filestack_image @user.filestack_url, transform: filestack_transform.resize(width:100, height:100).flip, id: 'someuniqueid' %>

[coveralls]

Coverage increased (+1.3%) to 97.537% when pulling aedfa68 on feature/FS-1343-add-image-tag into 6013a3d on develop.