use correct build date

[mwarning]

Before, the last modified date of the profiles.json was used as build date.

This is probably no a good idea either. We better use the last_modified header filed of the image and query that in the browser.

[mwarning]

54

[mwarning]

There are three ways to get a build date.

1. From a date field in the overview.json
2. Live via last_modified http header of the image
3. last_modified http header of overview.json

@aparcar what do you think is the best way?

[mwarning]

ok, I implemented variant 1.

The only drawback i see, is that now there is a date field for every model. But it should be easily compressible.

[mwarning]

fixed