Add reverse_www filter to fix www_redirect

[fullyint]

@QWp6t pointed out that when SSL is enabled, wordpress-site.conf.j2 will fail to forward http://www.example.com => https://example.com. The current template produces the conf below.

server {
  listen 80;
  server_name example.com;
  return 301 https://$host$request_uri;
}

server {
  listen 443 ssl http2;
  server_name www.example.com;
  return 301 $scheme://example.com$request_uri;
}

• The first server block doesn't match http://www.example.com because it doesn't include a match for www.
• The second server block doesn't match http://www.example.com because it listens only on 443.

This PR enables the first server block to also match www, taking the first step of forwarding http://www.example.com => https://www.example.com. Then the second server block will pick it up and strip the www.

I created and implemented a custom reverse_www filter to make the templating simpler.

[swalkinshaw]

Not sure what idiomatic Python is but seems like this should be under the else condition.

[fullyint]

Excellent, thank you. I see this in PEP 08:

# No
def bar(x):
    if x < 0:
        return
    return math.sqrt(x)

# Yes
def foo(x):
    if x >= 0:
        return math.sqrt(x)
    else:
        return None

Another improvement is that although re.match() will...

Return None if the string does not match the pattern

I should probably change the evaluation to if match is not None: because PEP 8 also says this:

beware of writing if x when you really mean if x is not None -- e.g. when testing whether a variable or argument that defaults to None was set to some other value. The other value might have a type (such as a container) that could be false in a boolean context!

[fullyint]

Additional commit

• code style better resembles python and Ansible conventions
• filter now accommodates one or more host names
• filter provides informative error message if input value is not string or list of strings

Pros of new filter

• makes the wordpress-site.conf.j2 template much more readable, sparing most users from seeing elaborate jinja filter sequences
• initiates the use of custom filters, bringing their power to Trellis

Cons of new filter

• adds far more lines of code overall than if we stick to existing filters
• probably wouldn't be used anywhere but in this instance, so it is not a benefit to DRY code

Below is an alternative to this PR's reverse_www filter. It may be the better solution for the www redirects issue at hand.

 {% if item.value.ssl is defined and item.value.ssl.enabled | default(False) %}
 server {
   listen 80;
-  server_name {{ item.value.site_hosts | join(' ') }};
+  server_name {{ item.value.site_hosts | join(' ') }}{% for host in item.value.site_hosts %} {{ (host[:3] == 'www.') | ternary(host[4:], 'www.' + host) }}{% endfor %};
   return 301 https://$host$request_uri;
 }
 {% endif %}

 {% for host in item.value.site_hosts if item.value.www_redirect | default(true) %}
 server {
   {% if item.value.ssl is defined and item.value.ssl.enabled | default(false) -%}
     listen 443 ssl http2;
   {% else -%}
     listen 80;
   {% endif -%}

-  server_name {{ host | match('^www\\.(.*)') | ternary(host | regex_replace('^www\\.(.*)', '\\1'), 'www.' + host ) }};
+  server_name {{ (host[:3] == 'www.') | ternary(host[4:], 'www.' + host) }};
   return 301 $scheme://{{ host }}$request_uri;
 }
 {% endfor %}