Create required sections

[zippy]

Some sections should be able to be marked as required, and this should be integrated into the process transition flow.

• add required field to Document that is a comma separated list of the sections that must be edited/filled
• add detection of when required sections have all been filled in, and make the "Mark process complete" button only become enabled when that has happened. This is detected by comparing the content for the section with the content in the initial entry of the document as it was created from the template and seeing if it's different.
• create a bit of UI that floats to the right of a document that displays a list of all the required sections and whether they have been completed.