Add ability to mark tags as deprecated

* Deprecated tags can't be added to posts, but existing deprecated tags
  in a post won't be removed
* Only empty tags can be marked as deprecated manually
* No tags can be manually undeprecated
** These limits don't apply to admins
* Deprecating or undeprecating a tag will create a new mod action to
  prevent people from going rogue
* Added deprecate/undeprecate commands for BURs
* Deprecating a tag via BUR removes all implications to and from it as well

app/views/wiki_pages/new.html.erb

@@ -18,6 +18,11 @@
   <% end %>
 
   <%= render "tag_relationships/alias_and_implication_list", tag: @wiki_page.tag %>
+
+  <% if @wiki_page.tag&.is_deprecated? %>
+    <p class="fineprint">This tag is <%= link_to "deprecated", wiki_page_path("help:deprecation_notice") %> and can't be added to new posts.</p>
+  <% end %>
+
   <%= render "wiki_pages/posts", wiki_page: @wiki_page %>
 <% end %>
 

app/views/wiki_pages/show.html.erb

@@ -37,6 +37,10 @@
     <% end %>
 
     <%= render "tag_relationships/alias_and_implication_list", tag: @wiki_page.tag %>
+
+    <% if @wiki_page.tag&.is_deprecated? %>
+      <p class="fineprint">This tag is <%= link_to "deprecated", wiki_page_path("help:deprecation_notice") %> and can't be added to new posts.</p>
+    <% end %>
   </div>
 
   <%= render "wiki_pages/posts", wiki_page: @wiki_page %>