Most significantly, corner masking wasn't working correctly pre-iOS 14.
This is because UIStackView's backing layer doesn't support
-cornerRadius. To work around this, we add the stack view to a wrapper
view. The corner radius is applied to that instead with masksToBounds
set true.
Adopted in GroupCallViewController. This changes how
ActionSheetController is built to have a single backdrop view. In the
translucentDark style, this backdrop is a UIVisualEffectView.
Creates a new enum, Theme.ActionSheet, to track different color
behaviors for action sheet themes.
* Pull out GroupMigrationActionSheet.
* Add group migration action sheets.
* Make changes to enable testing migrations.
* Rewrite banners in Swift.
* Refine group migration logic; add support for testability.
* Add legacy group upgrade action sheet.
* Add 'group update info messages' around migrations.
* Add new limits around group size.
* Rename migration modes to "manual", "auto-migration."
* Add legacy groups action sheets.
* Try to fill in missing UUIDs before migrating groups.
* Ignore "known unregistered" users during group migrations.
