feat(developer): Refactor coding guidelines and update frontend code

style

Update to living coding standard for frontend, with proper do's and
dont's.
Separated the coding guidelines into frontend and backend (PHP).

Signed-off-by: Ferdinand Thiessen <opensource@fthiessen.de>

developer_manual/getting_started/coding_standards/php.rst

@@ -0,0 +1,157 @@
+====================
+PHP coding standards
+====================
+
+Starting with Nextcloud 19 there is a shared `PHP Coding Standards Fixer <https://github.com/FriendsOfPhp/PHP-CS-Fixer>`_ configuration you can use to automatically format your app's source code. For full details see the `repository on GitHub <https://github.com/nextcloud/coding-standard/>`_.
+
+Always use::
+
+  <?php
+
+at the start of your php code. The final closing::
+
+  ?>
+
+should not be used at the end of the file due to the `possible issue of sending white spaces <https://stackoverflow.com/questions/4410704/php-closing-tag>`_.
+
+Comments
+--------
+All API methods need to be marked with `PHPDoc <https://en.wikipedia.org/wiki/PHPDoc>`_ markup. An example would be:
+
+.. code-block:: php
+
+  <?php
+
+  /**
+   * Description what method does
+   * @param Controller $controller the controller that will be transformed
+   * @param API $api an instance of the API class
+   * @throws APIException if the api is broken
+   * @since 4.5
+   * @return string a name of a user
+   */
+  public function myMethod(Controller $controller, API $api) {
+    // ...
+  }
+
+Objects, functions, arrays & variables
+--------------------------------------
+
+Use *UpperCamelCase* for Objects, *lowerCamelCase* for functions and variables. If you set
+a default function/method parameter, do not use spaces. Do not prepend private
+class members with underscores.
+
+.. code-block:: javascript
+
+  class MyClass {
+
+  }
+
+  function myFunction($default=null) {
+
+  }
+
+  $myVariable = 'blue';
+
+  $someArray = array(
+      'foo'  => 'bar',
+      'spam' => 'ham',
+  );
+
+  ?>
+
+
+Operators
+---------
+
+Use **===** and **!==** instead of **==** and **!=**.
+
+Here's why:
+
+.. code-block:: php
+
+  <?php
+
+  var_dump(0 == "a"); // 0 == 0 -> true
+  var_dump("1" == "01"); // 1 == 1 -> true
+  var_dump("10" == "1e1"); // 10 == 10 -> true
+  var_dump(100 == "1e2"); // 100 == 100 -> true
+
+  ?>
+
+Control structures
+------------------
+
+* Always use { } for one line ifs
+* Split long ifs into multiple lines
+* Always use break in switch statements and prevent a default block with warnings if it shouldn't be accessed
+
+.. code-block:: php
+
+  <?php
+
+  // single line if
+  if ($myVar === 'hi') {
+      $myVar = 'ho';
+  } else {
+      $myVar = 'bye';
+  }
+
+  // long ifs
+  if (   $something === 'something'
+      || $condition2
+      && $condition3
+  ) {
+    // your code
+  }
+
+  // for loop
+  for ($i = 0; $i < 4; $i++) {
+      // your code
+  }
+
+  switch ($condition) {
+      case 1:
+          // action1
+          break;
+
+      case 2:
+          // action2;
+          break;
+
+      default:
+          // defaultaction;
+          break;
+  }
+
+  ?>
+
+Unit tests
+----------
+
+Unit tests must always extend the ``\Test\TestCase`` class, which takes care
+of cleaning up the installation after the test.
+
+If a test is run with multiple different values, a data provider must be used.
+The name of the data provider method must not start with ``test`` and must end
+with ``Data``.
+
+.. code-block:: php
+
+    <?php
+    namespace Test;
+    class Dummy extends \Test\TestCase {
+        public function dummyData() {
+            return array(
+                array(1, true),
+                array(2, false),
+            );
+        }
+
+        /**
+         * @dataProvider dummyData
+         */
+        public function testDummy($input, $expected) {
+            $this->assertEquals($expected, \Dummy::method($input));
+        }
+    }