opportunity: accept meetup without date or location

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
opportunity: prettier meetup creation

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
opportunity: prettier meetup participation form

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
opportunity: by default the server uses port 8080

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
opportunity: support not being at web server root

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
opportunity: meetups hash also rely on timestamp

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
examples: update xymus.net nitcorn config to include an opportunity client

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>