Fixes #17228 - Select templates as global defaults (#4283)

This should allow user to select templates that will be used
for building PXE Default on TFTP proxy. These templates will
not be updated from seeds during updates.

Fixes #12543 - enable utf-8 encoding in email address

According to RFC 6532, the local part of an email address
can be non-ascii characters, and should support in utf-8.
The current email validation doesn't allow it,
so emails like Pelé@example.com and 我買@example.com are not supported

fixes #16725 - add plugin extension point for smart proxies

Fixes #15370 - Validate settings values

Adds validations for administrator, email_reply_address
and trusted_puppetmaster_hosts settings

Fixes #14198 - remote_addr validation accepts regex with digit matching

Fixes #6924 - Additional validations for Setting

Fixes #12698 - Insufficient URL validation Smart Proxy and Medium.

Problem: The regex that validates smart proxies URLs only matches
'beginning of text'. This allows us to add just \n after a valid URL and
put anything after it. For instance, javascript:alert('hacked'). I...

Refs #3809 - Remove cops for empty lines

Fixes #8414 - extracts validators from models

Also cleans up some unused validators looking for trailing whitespace
that is stripped by the StripWhitespaces concern.
Tried to make some sense from unmatching regexps and validation error
messages. Some validations might need a different regexp to check for what...