Clean up sequence editing form. Create new CSS for sequence editing.
Add thumnail_url to sequences.  Add help-text and preview to thumbnail

[#69218820] Add info below thumbnail URL to indicate correct size of
thumbnail image that should be linked to.

[#69218026] An author can add a thumbnail to a **sequence** in LARA and
publish to the portal so it shows up in the Browse Instructional materials listing

https://www.pivotaltracker.com/story/show/69218026
https://www.pivotaltracker.com/story/show/69218820