Add series to search results

[owi92]

This adds series to search results.

Couple of notes:

• I don't really like the icon chosen for this, but the one I would prefer is not yet included in the icon package used in Tobira.
• The box shadow for the thumbnails is also debatable. I left it in so you can play around with it and see if you can find a nicer looking one.
• The series entries look samewhat empty when there is no description, but we discussed adding some metadata like a (potentially shortened) list of creators of events in that series and maybe also/alternatively direct links to the first few events, but this is tbd in another PR.

Closes #360

[LukasKalbertodt]

Regarding shadow: I like it, and could even imagine making it stronger. Same offset, blur of 6px, opacity of 40% for example