polybar: Add pop-up calendar to date module

Added calendar (using gsimplecal) which appears on left-clicking the
date module. Previous long date format switching has been moved to
right-click.

Under the hood, the official date module has been exchanged with a
custom script which simply replicates the functionality but allows
arbitrary commands to be executed on interactions with the module (since
it is now a script module).

nvim/.config/nvim/spell/en.utf-8.add

@@ -100,3 +100,4 @@ flexicurity
 indices
 Anthropocene
 ecotourism
+legitimation