Only accept "next" from current user

[sos4nt]

morgenbot should only accept a !next from the current user. Otherwise, the current user could accidentally be skipped if someone else types !next. In fact, !skip should be used for the latter and morgenbot could respond with an appropriate help message if someone tries to !next someone else.

☕ morgenbot

@john, you're up

👨 marc

Fixed some bugs
!next

☕ morgenbot

@marc it's not your turn. Use !skip to skip @john.