Smooth out various transaction/editing behaviors for Calendar

Summary:
Ref T11809.

• Allow users to remove the "Until" date from recurring events.
• When removing "Until", show a sensible string ("...set this event to repeat forever.")
• When users go through the "Make Recurring" workflow, don't require them to explicitly select "Recurring: Recurring" from the dropdown. This intent is clear from clicking "Make Recurring".
• When editing "All Future Events", don't literally apply date changes to them, since that doesn't make sense. We update the template, then reschedule any events which haven't been edited already. I think this is what users probably mean if they make this edit.
• When creating an event with a non-default icon, don't show "alice changed the icon from Default to Party.".
• Hide the "recurring mode" transaction, which had no string ("alice edited this Event.") and was redundant anyway.
• Also, add a little piece of developer text to make hunting these things down easier.

Test Plan: Edited various events, parents, children, made events recur, set until, unset until, viewed transactions, rescheduled parents, rescheduled children.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11809

Differential Revision: https://secure.phabricator.com/D16796