Consistent naming of Massmessage (vs. MassMessage)

see discussion in #259.
Fix docstring
Fix migration script
Use PEP 8 conforming empty list check
Fix single value check
Require global.message if not massmessage not used in instance context
Adjust massmessage sender address options

Use `adhocracy.email.from` instead of `adhocracy.email.noreply`, and
only offer this option if the system configuration allows this.

Furthermore add an explanation on why a certain sender option is
disabled.