Improve 'Create Assignment' process with better messaging

[shaunanoordin]

PR Overview

The Create Assignment process needs better messaging so Teachers can better understand what's happening at every stage of the process, and what they need to do next.

Notable changes in this PR are:

• The Create Assignment page tells users if they need to Select Images (if they haven't already) or if they can Select New Images (if they have images already)
• The View/Edit Assignment page has been changed so "Use the map to choose a set of images for your students to identify..." message no longer appears (because users can't choose new images)
• When submitting the Create/Edit Assignment request, the Submit button now turns into the spinning cog icon.
• Aside: the tutorial for Educators & Students have somewhat improved CSS, to allow for larger tutorial images.

PR Details

Create Assignment: when user is just creating an Assignment, the "Select Images" button becomes a bit more clear.

Create Assignment: when user has already selected images for the Assignment, the message for "Select Images" changes accordingly.

Create Assignment: when submitting the Assignment to the Education API server, the Submit buttons turns into an "I'm loading..." spinning icon.

View/Edit Assignment: when viewing an existing Assignment, the inappropriate "Select Images" messages disappears altogether.

Status

Ready for review

[simoneduca]

I left a comment. Other than that, LGTM.