Fixes issue where squad ignores its destination

Fixed - Squad doesn't move to destination if the destination was set
after the squad engaged in combat.
Squad rally points no longer get stuck behind towers

Created a separate script that contains the clickable area of a tower.
This allows the tower's unit interaction collider and player interaction
collider to be separated. Additionally, the legal spots for the rally
points no longer overlaps the tower's selectable area.
Moved UI Prefab to playable levels root directory

The UI prefab is shared between all playable levels.
InGameMenus Prefab no longer nested under UI Prefab

Updating the InGameMenus prefab doesn't automatically update the UI
prefab.