Post Workbench transition improvements

[anenadic]

• Move over to using Mermaid within the lesson once it is enabled in the workbench (link to
  current MR for that)
• Add alt-text to all images and consider captioning some of them. The caption is now in the
  place where the alt-text used to be, and the alt-text is in an extra attribute after the
  definition of the image (e.g. caption text{alt='Alt text'})
• Consider creating learner profiles
• The "More" drop down to get to learner extras moves into the hamburger side menu at the
  bottom when the screen width is reduced beyond a certain limit. Doug and I felt this wasn't great
  UX and could possibly be improved.