Add Foreman 1.4 docs
Empty release notes for 1.4
Add pre-sprint 16 release notes
Add sprint 16 release notes
Add usergroup cycle membership warning
added plugins section to the manual
Add sprint 17 release notes

TODO documentation for: #3312, #3699, #3047, #3732
fixed comments on plugins section.
Removed references to the legacy mysql package/adapter
1.3 -> {{page.version}} on some pages
Add sprint 18 release notes

TODO documentation for: db:seed, libvirt/dnsmasq providers, REMOTE_USER attrs(?), mysql/mysql2, Chef, proxy custom command