Make preview of upcoming talks shorter

[Anne-Victoria]

The preview of upcoming talks is too big. We should only show a short teaser (e.g. title, first sentence) and hide the rest unless the user expands it.

[Anne-Victoria]

Long description of upcoming talk is now removed and only title and abstract are shown. Full description of upcoming talks can be read on pretalx anyways. Some day we might enable expanding the description directly so that users don't have to switch to pretalx to read the full description.