Move scripts to Nix files

Move the scripts I commonly use into Nix files managed by Home Manager.

Some, like `mounter` and `unmounter` are desktop-only, but this can be
addressed in a future commit to create per-host scripts or
desktop/non-desktop scripts.

This commit also removes a number of unused scripts that are either
included in packages like `git-extras` or aren't used.

modules/home-manager/cli/scripts/move-firefox-screenshots.nix

@@ -0,0 +1,15 @@
+{ pkgs }:
+
+pkgs.writeShellApplication {
+  name = "move-firefox-screenshots";
+
+  runtimeInputs = with pkgs; [ findutils ];
+
+  text = ''
+    original_directory="''${HOME}/Downloads"
+    new_directory="''${HOME}/Pictures/Screenshots"
+
+    find "''${original_directory}" -mindepth 1 -maxdepth 1 -type f -name "Screenshot *" \
+      -exec mv {} "''${new_directory}" \;
+  '';
+}