Refactor tile cell borders aspect to use standard layout API.

Override conventional layout and cell methods.
Factor out a custom TileLayoutAttributes class to
use instead of UIEdgeInsets and initial constraint
constants. Keep the implementation detail of
borderInsets to TileCell only.
Fixup allowing EventViewController to update with pre-existing event.

Harden is-editing check. Refactor into a computed getter.
Tweak day cell layout.
Add app icon assets.