Lightning talks

[Chilipp]

This PR revises the text for the posters team and adds info about the lightning-talk session in January.

• revised text: https://1594-267395254-gh.circle-artifacts.com/0/SORSE/programme/posters/index.html
• news post: https://1594-267395254-gh.circle-artifacts.com/0/SORSE/programme/posters/session/index.html
• added submission deadline: https://1594-267395254-gh.circle-artifacts.com/0/SORSE/programme/call-for-contributions/index.html#next-submission-deadlines

One remaining question is though, whether we can turn the submission deadline from 9AM UTC to midnight UTC, as this is also the deadline for all the other submissions. Would that work @florianthiery?

closes #392

[Chilipp]

alright, the deadline has been changed to January 10th, midnight UTC. And I also added the submission deadline to the links. Merging this as Louise gave her OK