update all js deps

This addresses this error travis-ci builds have been hitting:

Error: Cannot find module '@babel/compat-data/corejs3-shipped-proposals'
https://travis-ci.org/github/scottlamb/moonfire-nvr/jobs/684922930
nodejs/node#32852 (comment)

package.json

@@ -26,12 +26,12 @@
     "clean-webpack-plugin": "^3.0.0",
     "compression-webpack-plugin": "^3.1.0",
     "css-loader": "^3.4.2",
-    "eslint": "^6.8.0",
+    "eslint": "^7.0.0",
     "eslint-config-google": "^0.14.0",
-    "file-loader": "^5.1.0",
-    "html-loader": "^0.5.5",
-    "html-webpack-plugin": "^3.2.0",
-    "prettier": "1.19.1",
+    "file-loader": "^6.0.0",
+    "html-loader": "^1.1.0",
+    "html-webpack-plugin": "^4.3.0",
+    "prettier": "2.0.5",
     "style-loader": "^1.1.3",
     "webpack": "^4.41.6",
     "webpack-cli": "^3.3.11",