Campaign | Challenges: Accept challenge invitation

[nscharioth]

Once one team has selected another team for a challenge in #308, the other team receives an invitation which they can either accept to decline. If the invitation is accepted the challenge becomes live and is shown in a different color on the teams page #180.

Prototype: https://www.figma.com/proto/7wE0Iveas3GGb8u4oL0714/App-Design-Mitglieder-App-(1)-(Copy)?node-id=2769-7270&t=b8swD5IOAFfZ0OSo-1&starting-point-node-id=2425%3A17429

Pollion reference: https://github.com/pollion/sunflower-app/issues/101

[nscharioth]

UX questions @leagerndt @JuliaFreitagBGSt :

• does only the team lead get challenge invitations or all team members?