Use warnings for some react eslint rules

Test plan:
    - All existing tests pass

flag=none

Refs DE-1426

Change-Id: I71482f85e89eeb62db45337cfb4b84c6ce9186d5
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/303812
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Ed Schiebel <eschiebel@instructure.com>
Reviewed-by: Jeffrey Johnson <jeffrey.johnson@instructure.com>
QA-Review: Aaron Shafovaloff <ashafovaloff@instructure.com>
Product-Review: Jeffrey Johnson <jeffrey.johnson@instructure.com>

.eslintrc.js

@@ -269,6 +269,15 @@ module.exports = {
         'import/order': 'off', // because it thinks 'jsx/whatever' and 'compiled/baz' should go in their groups. we don't want to encourage people to do that just so they move them back together once  those everything is in same dir
         'import/no-unresolved': 'off',
         'import/no-webpack-loader-syntax': 'off',
+
+        // TODO: resolve and remove these overrides
+        'react/no-string-refs': 'warn',
+        'react/no-this-in-sfc': 'warn',
+        'react/prop-types': 'warn',
+        'react/no-access-state-in-setstate': 'warn',
+        'react/no-find-dom-node': 'warn',
+        'react/no-unused-prop-types': 'warn',
+        'react/prefer-stateless-function': 'warn',
       },
     },
     {