**HSTS Preloading:**
In its strongest and recommended form, the [HSTS policy](https://www.chromium.org/hsts) includes all subdomains, and indicates a willingness to be “preloaded” into browsers:
`Strict-Transport-Security: max-age=31536000; includeSubDomains; preload`
**X-Xss-Protection:**
`1; mode=block` which tells the browser to block the response if it detects an attack rather than sanitising the script.
Not sure if it breaks with them or not, but no other directive
uses quotes and the nginx docs show examples without quotes,
so we're being consistent with all of that.
The different configurations are now all lower case, for consistent
naming.
`matrix_nginx_proxy_ssl_config` is now called
`matrix_nginx_proxy_ssl_preset`. The different options for "modern",
"intermediate" and "old" are stored in the main.yml file, instead of
being hardcoded in the configuration files. This will improve the
maintainability of the code.
The "custom" preset was removed. Now if one of the variables is set, it
will use it instead of the preset. This will allow to mix and match more
easily, for example using all the intermediate options but only
supporting TLSv1.2. This will also provide better backward
compatibility.
A new variable called `matrix_nginx_proxy_ssl_config` is created for
configuring how the nginx proxy configures SSL. Also a new configuration
validation option and other auxiliary variables are created.
A new variable configuration called `matrix_nginx_proxy_ssl_config` is
created. This allow to set the SSL configuration easily using the
default options proposed by Mozilla. The default configuration is set to
"Intermediate", removing the weak ciphers used in the old
configurations.
The new variable can also be set to "Custom" for a more granular control.
This allows to set another three variables called:
- `matrix_nginx_proxy_ssl_protocols`,
- `matrix_nginx_proxy_ssl_prefer_server_ciphers`
- `matrix_nginx_proxy_ssl_ciphers`
Also a new task is added to validate the SSL configuration variable.
Fix regression since 2a50b8b6bb (#597).
Dimension is intended to be embedded in various clients,
be it the Element service that we host (at element.DOMAIN),
some other Element (element-desktop running locally), etc.
