Rename all settings to camelCase

The reason behind this change is that it is harder
to type a hyphen than press SHIFT.
Refactor "mail" configuration section
Move getWaitingRoomLocation to LogItCore
Change default value for globalPassword.invalidateAfter to "5min"
Change order of on-login username checks
Change Polish translation of usernameInvalid
Add regex username validation to "/logit account rename"