modernize js and use babel (go-gitea#8973)

* modernize js and use babel

- add babel toolchain to transform modern JS to ES5
- extend eslint config for modern rules
- fixes linting issues via `eslint --fix` and manual fixes

* run 'make css' to satisfy CI

* code style tweaks and set js indendation to 2 in .editorconfig

* regenerate js

.editorconfig

@@ -24,7 +24,7 @@ indent_size = 2
 
 [*.js]
 indent_style = space
-indent_size = 4
+indent_size = 2
 
 [Makefile]
 indent_style = tab

.eslintrc

@@ -1,28 +1,51 @@
 root: true
 
 extends:
+  - eslint-config-airbnb-base
   - eslint:recommended
 
 parserOptions:
-  ecmaVersion: 2015
+  ecmaVersion: 2020
 
 env:
   browser: true
-  jquery: true
   es6: true
+  jquery: true
   node: true
 
 globals:
   Clipboard: false
   CodeMirror: false
+  Dropzone: false
   emojify: false
+  hljs: false
   SimpleMDE: false
-  Vue: false
-  Dropzone: false
   u2fApi: false
-  hljs: false
+  Vue: false
 
 rules:
-  no-unused-vars: [error, {args: all, argsIgnorePattern: ^_, varsIgnorePattern: ^_, ignoreRestSiblings: true}]
-  prefer-const: [2, {destructuring: all}]
+  camelcase: [0]
+  comma-dangle: [2, only-multiline]
+  consistent-return: [0]
+  default-case: [0]
+  func-names: [0]
+  max-len: [0]
+  newline-per-chained-call: [0]
+  arrow-body-style: [0]
+  no-alert: [0]
+  no-continue: [0]
+  no-mixed-operators: [0]
+  no-multi-assign: [0]
+  no-new: [0]
+  no-param-reassign: [0]
+  no-plusplus: [0]
+  no-restricted-syntax: [0]
+  no-shadow: [0]
+  no-unused-vars: [2, {args: all, argsIgnorePattern: ^_, varsIgnorePattern: ^_, ignoreRestSiblings: true}]
+  no-use-before-define: [0]
   no-var: [2]
+  one-var-declaration-per-line: [0]
+  one-var: [0]
+  prefer-const: [2, {destructuring: all}]
+  prefer-destructuring: [0]
+  radix: [2, as-needed]