Enable image embedding on create issue form

Today I wanted to embed an image while also creating a new issue, I'm sure others have wanted the same. This implements that, feedback please because I'm sure this can be better.

Longer term it would be cool if the user could control whether the image is embedded in the comment or issue summary when using the form on the issue node (my only idea so far is to add two huge buttons). Shorter term I think I'd like to implement #28 because we have the code for that already.