Slide overlay

These MRs should be merged first:

• https://git.uwaterloo.ca/science-computing/Scinage/merge_requests/43
• !11 (closed)
• !7 (closed)

Once the above MRs are merged, the base of this MR should be changed to master

Summary

This MR adds the initial implementation of the slide overlay feature

Users can now go to edit slideshow and add a slide as a slide overlay. This slide will render above all the other slides in a slideshow and will render without a transparent background.

Also makes the following changes:

• Adds a new overlay parameter to slides
  • If true: renders a slide without a background color
• Adds new DB migration to add a new field to the Slideshow model ("overlay") which is an optional foreign key to a slide
• Add ability to edit slideshow overlay on the edit slideshow page

Other considerations (not in scope for this MR)?

• Having multiple slide overlays?
• Making the edit slideshow page nicer

Click to see an example in action

Screenshots