Allow upload quotas to be set on a per-account basis.

By writing files called "quota-megabytes" and "quota-files" in the
account directory, a server can now override the default limits (100MB
and 100 files) for specific accounts.
go fmt
Modified the osx instructions

Among the other issues discovered when installing pond using the osx
instructions: the gtk+3 settings have to be applied before installing
gtk+3 or you will get errors.
cli: improve delete command

 - add support for deleting drafts, received, and sent messages
 - require confirmation for deleting messages as well as contacts
 - doesn't delete pending outbound messages - they must be aborted instead
threat model: two more things contacts can do
cli: correct table heading for help --all
Added system.IsSafe() for OSX; editstate now runs on OSX
Hopefully clarify the difference between GTK+ Quartz and XQuartz.
Merge branch 'savagejen-master'
Move (dont-)retain commands to non-subprocess switch.
Merge branch 'leif-master'
Avoid casting in type switch and don't crash when deleting drafts without a destination.
Merge branch 'leif-cli-delete-command'
Merge branch 'leif-threat-model'
Merge branch 'leif-cli-help'