Actually fix lock file issues (hopefully)

Undid all the changes to the Gemfile and Gemfile.lock, and readded
rubocop. This seems to work, so hopfully CI passes.

Gemfile.lock

@@ -332,7 +332,7 @@ GEM
       rubocop-ast (>= 0.5.0)
       ruby-progressbar (~> 1.7)
       unicode-display_width (>= 1.4.0, < 2.0)
-    rubocop-ast (0.7.1)
+    rubocop-ast (0.8.0)
       parser (>= 2.7.1.5)
     rubocop-rails (2.8.1)
       activesupport (>= 4.2.0)
@@ -354,6 +354,7 @@ GEM
       tilt
     sdoc (1.1.0)
       rdoc (>= 5.0)
+    semantic_range (2.3.0)
     simple_form (5.0.2)
       actionpack (>= 5.0)
       activemodel (>= 5.0)
@@ -394,7 +395,8 @@ GEM
     webpacker (5.2.1)
       activesupport (>= 5.2)
       rack-proxy (>= 0.6.1)
-      railties (>= 4.2)
+      railties (>= 5.2)
+      semantic_range (>= 2.3.0)
     websocket-driver (0.7.3)
       websocket-extensions (>= 0.1.0)
     websocket-extensions (0.1.5)