api keys: add API key usage tracking.

Track when an API key was last used, which IP address last used it, and
how many times it's been used overall.

This is so you can tell when an API key was last used, so you know if
the key is safe to delete, and so you can tell if an unrecognized IP has
used your key.

app/views/api_keys/index.html.erb

@@ -51,14 +51,22 @@
           <%= safe_join(api_key.permitted_ip_addresses, "<br>".html_safe).presence || "All" %>
         <% end %>
 
-        <% if !params[:user_id].present? %>
-          <% t.column "User" do |api_key| %>
-            <%= link_to_user api_key.user %>
+        <% t.column :uses %>
+
+        <% t.column "Last Used" do |api_key| %>
+          <%= time_ago_in_words_tagged api_key.last_used_at %>
+
+          <% if api_key.last_ip_address.present? %>
+            <br>by <%= api_key.last_ip_address %>
           <% end %>
         <% end %>
 
         <% t.column "Created" do |api_key| %>
           <%= time_ago_in_words_tagged api_key.created_at %>
+
+          <% if !params[:user_id].present? %>
+            <br> by <%= link_to_user api_key.user %>
+          <% end %>
         <% end %>
 
         <% t.column column: "control" do |api_key| %>