deps: Update [@rails/]webpacker to 5.4.0

Also ran `rails webpacker:install` in the Docker container.

This ensures the packages in `package[-lock].json` and
`Gemfile[.lock]` are in sync with the `webpacker` Ruby gem.

Had to review (and decline) certain conflicting file updates.

Gemfile.lock

@@ -261,7 +261,7 @@ GEM
       rack (>= 2.0.0)
     rack-jsonp (1.3.1)
       rack
-    rack-proxy (0.6.5)
+    rack-proxy (0.7.0)
       rack
     rack-test (1.1.0)
       rack (>= 1.0, < 3)
@@ -353,7 +353,7 @@ GEM
       tilt
     sdoc (1.1.0)
       rdoc (>= 5.0)
-    semantic_range (2.3.0)
+    semantic_range (3.0.0)
     simple_form (5.0.2)
       actionpack (>= 5.0)
       activemodel (>= 5.0)
@@ -391,7 +391,7 @@ GEM
       addressable (>= 2.3.6)
       crack (>= 0.3.2)
       hashdiff (>= 0.4.0, < 2.0.0)
-    webpacker (5.2.1)
+    webpacker (5.4.0)
       activesupport (>= 5.2)
       rack-proxy (>= 0.6.1)
       railties (>= 5.2)

package.json

@@ -1,17 +1,19 @@
 {
   "dependencies": {
     "@rails/ujs": "^6.0.2",
-    "@rails/webpacker": "^5.2.1",
+    "@rails/webpacker": "5.4.0",
     "bootstrap": "^4.1.1",
     "jquery": "^3.3.1",
     "jquery-ujs": "^1.2.2",
     "popper.js": "^1.14.3",
     "rails-erb-loader": "^5.5.2",
     "swagger-ui": "^3.19.2",
+    "webpack": "^4.46.0",
+    "webpack-cli": "^3.3.12",
     "yarn": ">=1.0.2"
   },
   "devDependencies": {
-    "webpack-dev-server": "^3.11.0"
+    "webpack-dev-server": "^3.11.2"
   },
   "resolutions": {
     "node-fetch": "^2.6.1",