Add documentation to general.yml parameters

Also available at http://alaveteli.org/docs/customising/config
Add default INCOMING_EMAIL_PREFIX

All our examples use the 'foi+' prefix so lets make this the default. If
people _really_ want no prefix, they can change the setting to ''. This
will probably cover the more general cases.
Emphasise suggestion to change emergency admin details
Don't set non-default value for USE_GHOSTSCRIPT_COMPRESSION

We don't set this to true as a default so don't supply the non-default
to new installs.
Set default MAX_REQUESTS_PER_USER_PER_DAY

We supply a default example for MAX_REQUESTS_PER_USER_PER_DAY so we
might as well set it as default and allow the user to tune if they
dislike the default.
Add default MTA_LOG_PATH

We default MTA_LOG_TYPE to exim so we can set the default log path too
Use default setting for MINIMUM_REQUESTS_FOR_STATISTICS

Defaults to 100 so use this in the example config
Provide sensible defaults for emails

Provide something that at least has a chance of existing and might get read
Update default RESPONSIVE_STYLING