Implement image upload for posts

[blutorange]

When writing a post, I can only insert a link to an image with an URL. Users should be able to upload their own images.

I can think of two ways to implement this:

• [x] Create an image upload dialog client-side, see 090-master.js, line 56-57
• [x] Create a servlet on the server-side accepting an image and returning a URL.

or alternatively:

• [ ] Convert the uploaded image to a base 64 string.
• [ ] Find a way to insert this directly into the markdown?

[blutorange]

http://www.dropzonejs.com/ might be nice

[blutorange]

Implemented now with da96b22acb5138e44957e1e935d2dab7c2b67359