Image feature UI, updates to backend image upload implementations, some validations and tests

[suksw]

A message in an alert json object will look like:

<h1>Heading</h1><p></p><p>Paragraph</p><ul><li><p>a bullet</p></li><li><p>another bullet</p></li></ul><ol><li><p>first point</p></li><li><p>second point</p></li></ol><p><img src=\"/api/images/image/16\"></p><p></p>

Note: the image url will be similar to /api/images/image/16. All requests to /api can be forwarded to the backend.