Initial ESLint setup

This sets up ESLint and its config, and disables a bunch of recommended
lints that currently do not pass. Later PRs/commits will fix those lints
and re-enable them.

.eslintrc.js

@@ -0,0 +1,77 @@
+module.exports = {
+    root: true,
+    extends: [
+        'eslint:recommended'
+    ],
+    env: {
+        es6: true
+    },
+    parserOptions: {
+        ecmaVersion: 'latest'
+    },
+    overrides: [
+        {
+            // Server-side files (plus this configuration file)
+            files: ["src/**/*.js", "server.js", ".eslintrc.js"],
+            env: {
+                node: true
+            }
+        },
+        {
+            // Browser-side files
+            files: ["public/**/*.js"],
+            env: {
+                browser: true,
+                jquery: true
+            },
+            parserOptions: {
+                sourceType: 'module'
+            },
+            // These scripts are loaded in HTML; tell ESLint not to complain about them being undefined
+            globals: {
+                DOMPurify: 'readonly',
+                droll: 'readonly',
+                Fuse: 'readonly',
+                Handlebars: 'readonly',
+                hljs: 'readonly',
+                localforage: 'readonly',
+                moment: 'readonly',
+                pdfjsLib: 'readonly',
+                Popper: 'readonly',
+                showdown: 'readonly',
+                showdownKatex: 'readonly',
+                SVGInject: 'readonly',
+                toastr: 'readonly'
+            }
+        }
+    ],
+    // There are various vendored libraries that shouldn't be linted
+    ignorePatterns: ['public/lib/**/*', '*.min.js', 'src/ai_horde/**/*'],
+    // Most, if not all, of these rules should eventually be enabled and the code changed. They're disabled so that
+    // linting passes.
+    rules: {
+        'no-unused-vars': 'off',
+        'no-useless-escape': 'off',
+        'no-control-regex': 'off',
+        'no-redeclare': 'off',
+        'no-async-promise-executor': 'off',
+        'no-inner-declarations': 'off',
+        'no-extra-semi': 'off',
+        'no-undef': 'off',
+        'no-prototype-builtins': 'off',
+        'no-unused-labels': 'off',
+        'no-extra-boolean-cast': 'off',
+        'require-yield': 'off',
+        'no-case-declarations': 'off',
+        'use-isnan': 'off',
+        'no-self-assign': 'off',
+        'no-unsafe-negation': 'off',
+        'no-constant-condition': 'off',
+        'no-empty': 'off',
+        'no-unsafe-finally': 'off',
+        'no-dupe-keys': 'off',
+        'no-irregular-whitespace': 'off',
+        'no-regex-spaces': 'off',
+        'no-fallthrough': 'off'
+    }
+};