DEV: Ability to collect stats without exposing them via API

[AndrewPrigorshnev]

This adds the ability to collect stats without exposing them among other stats via API.

The most important thing I wanted to achieve is to provide an API where stats are not exposed by default, and a developer has to explicitly specify that they should be exposed (expose_via_api: true). Implementing an opposite solution would be simpler, but that's less safe in terms of potential security issues.

When working on this, I had to refactor the current solution. I would go even further with the refactoring, but the next steps seem to be going too far in changing the solution we have, and that would also take more time. Two things that can be improved in the future:

1. Data structures for holding stats can be further improved
2. Core stats are hard-coded in the About template (it's hard to fix it without correcting data structures first, see point 1):
   

   discourse/app/views/about/index.html.erb

   Lines 61 to 101 in 63a0700

   	<tr>
   	<th>&nbsp;</th>
   	<th><%=t 'js.about.stat.all_time' %></th>
   	<th><%=t 'js.about.stat.last_day' %></th>
   	<th><%=t 'js.about.stat.last_7_days' %></th>
   	<th><%=t 'js.about.stat.last_30_days' %></th>
   	</tr>
   	<tr>
   	<td class='title'><%=t 'js.about.topic_count' %></td>
   	<td><%= stats["topic_count"] %></td>
   	<td><%= stats["topics_last_day"] %></td>
   	<td><%= stats["topics_7_days"] %></td>
   	<td><%= stats["topics_30_days"] %></td>
   	</tr>
   	<tr>
   	<td><%=t 'js.about.post_count' %></td>
   	<td><%= stats["post_count"] %></td>
   	<td><%= stats["posts_last_day"] %></td>
   	<td><%= stats["posts_7_days"] %></td>
   	<td><%= stats["posts_30_days"] %></td>
   	</tr>
   	<tr>
   	<td><%=t 'js.about.user_count' %></td>
   	<td><%= stats["user_count"] %></td>
   	<td><%= stats["users_last_day"] %></td>
   	<td><%= stats["users_7_days"] %></td>
   	<td><%= stats["users_30_days"] %></td>
   	</tr>
   	<tr>
   	<td><%=t 'js.about.active_user_count' %></td>
   	<td>&mdash;</td>
   	<td><%= stats["active_users_last_day"] %></td>
   	<td><%= stats["active_users_7_days"] %></td>
   	<td><%= stats["active_users_30_days"] %></td>
   	</tr>
   	<tr>
   	<td><%=t 'js.about.like_count' %></td>
   	<td><%= stats["like_count"] %></td>
   	<td><%= stats["likes_last_day"] %></td>
   	<td><%= stats["likes_7_days"] %></td>
   	<td><%= stats["likes_30_days"] %></td>

The most significant refactorings are:

1. Introducing the Stat model
2. Aligning the way the core and the plugin stats' are registered

[discoursebot]

This pull request has been mentioned on Discourse Meta. There might be relevant details there:

https://meta.discourse.org/t/shields-io-unable-to-retrieve-discourse-statistics-api/128574/12