[#534] Make reST labels for config settings consistent

Make the reST label for cross-referencing each config setting the same
as the name of the config setitng itself, e.g.
ckan.activity_streams_email_notifications, don't mix and match different
styles of ., _ and - like ckan-activity-streams-email-notifications,
ckan_activity_streams_email_notifications, etc.
[#534] Add reST labels to all config options

Now we can easily cross-reference any config option from any file in
docs/.
[#534] Edit top of configuration docs page

Use the default ini file locations; Clarify what the [app:main] section
is; say "restart webserver" not Apache, webserver may not be Apache.
[#534] Use .. deprecated:: in config docs