Add free text field to version to support sha based ids

[mmohrhard]

This allows us for example to support daily builds.

[mmohrhard]

This should be added to base/model.py's Version class. And then we need to make sure that the change is used all over the place. Currently we just use the version string.

I think a nullable string is the best solution and add it to the unique requirement as well as changing the str_without_product method and the other methods that query a version to also support the sha based stuff.