Code inputs are contenteditable

[tonyfast]

this pull request begins steps towards considering cells inputs to be edittable objects. the tabbing between code cell inputs was not desirable, but some of the following suggestions were made.

• “Since there’s no editable box, it’s not going to take me. This should work with an editable box.”
• Compares the cells to input boxes in a form (ie. google forms).
• “Autofocus mode” where the focus jumps immediately to a editable field when you land on a page.
• Does think this would work even if it’s not editable because code cells were editable. It’s difficult to find by output or input.
• Headings are good, but I feel that code blocks are edit boxes. Even if they are read-only edit boxes. They aren’t plain text.

these changes add a flag to cell_contenteditable that simulates code inputs as content-editable objects. i'll probably keep workin this some more.

@Jenneh @smythp @isabela-pf could y'all scope out https://iota-school.github.io/notebooks-for-all/exports/Imaging_Sky_Background_Estimation-contenteditable-code.html and see if this worth sending to our testers for A/B tests?

[tonyfast]

i merged in #43 so i can add the content to the docs

[tonyfast]

this revision has turned into a total nightmare. the intent of the pull requests changed. closing this in favor of #44

[smythp]

👍