[SqlLab] Fix a few UI issues (#4401)

1. tab alignment css
2. if tabs are more than 1 row, need to calculate content area height
3. clean up height calculation.

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

@@ -15,7 +15,7 @@ describe('SqlEditor', () => {
     latestQuery: queries[0],
     tables: [table],
     queries,
-    height: '',
+    getHeight: () => ('100px'),
     editorQueries: [],
     dataPreviewQueries: [],
   };