[iOS] Relative dates improvements

[sdaitzman]

Relative dates are a mobile platform enhancement that allows users to enable a more human-friendly view of assignment dates e.g. some learning assignment may be displayed as due "Today" or "3 days ago" or "October 1" rather than always showing full dates.

Future enhancements should be made based on the following considerations:

• Events within the next e.g. 24-48 hours (or longer, or recently due) may benefit from including time display
• Ensure assignments due very early in the local timezone e.g. 1:15 AM are not missed
• Since the next 7 days are currently shown using weekday specifiers, consider when to switch to "Next ___day" e.g. for "Monday," consider the case of an assignment due "Monday" in 1 week
• Internationalization and date conventions e.g. date formatting order, what counts as "Next Monday" vs "Monday" etc.
• It very likely makes sense to switch to relative date display using a higher number of days before falling back to dates e.g. "In 12 days"
• Consider a hybrid display e.g. "July 28 - in 2 days"
• Consider allowing more user customization of date displays

Matching Android roadmap ticket: openedx/openedx-app-android#370