Add server block customizations 

This allows users to add a file /etc/nginx/conf.d/includes/mydomain.com.conf, the contents of which will be included in the server block for mydomain.com.
Move custom server blocks to STORAGE_ROOT
test_domain -> domain
prettify mail guide
update comments in backup.py
create STORAGE_ROOT/backup/duplicity if it doesn't exist

fixes #158
show custom DNS records in the control panel too, fixes #155
document the DNS API in the control panel

see #140, #155, df20d447a9c2c41f92f2168a7a0e8aec9ea47e5a
bug in the IPV6 question
normalize some whitespace
if there are no admins when trying to access the control panel, tell the user how to make an admin from SSH
fix link to /cloud in the admin, fixes #160
Merge pull request #156 from skosch/patch-1

Allow users to insert custom nginx configuration directives through new optional files.
Replace spaces by tabs in 106-109
Merge pull request #162 from skosch/patch-2

Replace spaces by tabs in 106-109. See 92c7815d2ca2c11aa0e671e942f07ad78c113f95.