Edit community profile

The following PR depends on #27, #29 and #30.

It implements the functionality of editing community profile. 
`CommunityForm` is a model form and hence can be initialized with an instance of that model.
It is a convenient way to fill the form with model fields and save new data.
[Documentation on model forms in Django](https://docs.djangoproject.com/en/dev/topics/forms/modelforms/#the-save-method)