Add TODOs

[cpina]

schema-collaboration has a comment system: data manager can write comments for the researchers, they can answer them or write new comments.

It has been suggested that adding TODOs would be interesting. This would be comments with a checkbox. The data manager could write something along the lines of:

• add units in the table schema
• add collaborators in the package

And the researchers could tick them to know what's left to do.

The TODO would have an writer and an owner. The owner could be the same person as the writer (for self-TODOs)