add button for feature combinations

[johenglisch]

The button exists now (see picture -- I haven't deployed it yet).

@HedvigS I'm not entirely sure where the button should actually go on the page. For now I put it at the bottom of the description section (right above the map). So, if you want me to move it somewhere else, just tell me.

Also, which features should be combinable here? All of them or only the ones listed in Related features (this branch does the former rn)?

@xrotwang The code in the template feels a bit weird (like, the part where I import DBSession into the template). Is there a more idiomatic way to do this that I'm missing?

[HedvigS]

Thanks @johenglisch !

Let's leave the position as is now, if anyone has opinions I'll figure out if they're worthwhile and let you know :)

All features should be combinable please :)

[johenglisch]

Thanks! So, as far as I'm concerned this is ready to be merged.