Relayout SQL Editor (#6872)

* Relayout SQL Editor

- Refactor SQL editor to remove usage of bootstrap col, row and collapse to simplify the layout
- Replace the react-split-pane libraray with react-split to allow custom styling of the gutter area without sacrifice correctness of the ace editor height calculation
- Rewrite the left pane animation via plain css transition and animate it to slide in and out
- General code and css clean up

* Smooth out the visual transition during dragging

(cherry picked from commit 19f82b729c7a939f12b1c5da6022c0fd76fa3ec9)

* Adjust how the height of the south pane is computed, fixing cypress tests

superset/assets/spec/javascripts/sqllab/SqlEditor_spec.jsx

@@ -38,6 +38,11 @@ describe('SqlEditor', () => {
     defaultQueryLimit: 1000,
     maxRow: 100000,
   };
+
+  beforeAll(() => {
+    jest.spyOn(SqlEditor.prototype, 'getSqlEditorHeight').mockImplementation(() => 500);
+  });
+
   it('is valid', () => {
     expect(
       React.isValidElement(<SqlEditor {...mockedProps} />),