AA-289: rewrite how course updates behave when dismissed

Previously, dismissing one course update would disable all future
updates. But that feels a bit limiting.

Instead, these are the new rules:
- If the newest update has not been dismissed yet, it gets displayed.
- If the newest update has been dismissed, we display nothing.
- Editing the newest update will cause it to be displayed again.
- New updates get displayed, even if previous updates were dismissed.

This commit includes a light refactoring and addition of some utility
methods to help manage the above.

It also notes that the learning MFE does not use the "latest_update"
waffle option, which controls the labeling of the update message.