Allow organizers to send a badge to a speaker

[friedger]

Organizers should be able to send a badge to a participant that gave a talk at the current event.

• Add button to send badge for organizers only
• Add screen for participant to accept
• Add screen for organizer to confirm acceptance
• Send badge to participant
• Add list of previous speakers to chapter details list

Use Firebase Auth and Nearby API.

Depends on #616

[PareshMayani]

This is fantastic feature and going to be interesting among participants and speakers.