Adds an integer "time_to_complete" to activities

[#49883833] This handles saving the time for the author, but doesn't display the value to end users; at this point the story doesn't describe where it should go. Note that the value is in integer minutes, so activities over an hour would need e.g. "90" instead of "1:30". We can parse that more effectively on display.

https://www.pivotaltracker.com/story/show/49883833