eslint: install eslint with front-end so it can be run manually

eslint can be run manually on the main .js files with:

  ./node_modules/.bin/eslint kallithea/public/js/base.js kallithea/public/js/codemirror_loadmode.js kallithea/public/js/graph.js

- but it is quite noisy. Some problems do however stand out as relevant to fix.

Script sections in HTML files can also be checked after removing mako markup:

  hg up -cr.
  sed -i -e 's/\${[^{}]*\({[^{}]*}[^{}]*\)*}/""/g' -e 's/%\(if\|else\|endif\|for\|endfor\)\>.*//g' -e 's/##.*//g' $(hg loc 'kallithea/templates/**.html')
  vim kallithea/templates/pullrequests/pullrequest.html +139  # blank out the multi line 'var url = ${}'
  ./node_modules/.bin/eslint $(hg loc 'kallithea/templates/**.html')
  hg up -Cr.

- but that is even more noisy.

The noise is mainly due to eslint not knowing that everything runs together,
with kallithea/templates/base/root.html defining global variables,
kallithea/public/js/base.js using these and defining functions, which then is
used "everywhere". There might be solutions to that - this is a starting point.

.eslintrc.js

@@ -0,0 +1,21 b''
+module.exports = {
+    "env": {
+        "browser": true,
+        "es6": true,
+        "jquery": true
+    },
+    "extends": "eslint:recommended",
+    "globals": {
+        "Atomics": "readonly",
+        "SharedArrayBuffer": "readonly"
+    },
+    "parserOptions": {
+        "ecmaVersion": 2018,
+        "sourceType": "module"
+    },
+    "plugins": [
+        "html"
+    ],
+    "rules": {
+    }
+};

kallithea/front-end/package.json

@@ -7,6 +7,7 b''
     "codemirror": "5.49.0",
     "datatables.net": "1.10.20",
     "datatables.net-bs": "1.10.20",
+    "eslint": "6.8.0",
     "jquery": "3.4.1",
     "jquery.caret": "0.3.1",
     "jquery.flot": "0.8.3",