Make conferences easier to click

Also unifies h1-style a bit (see "My Assignments").

app/views/conferences/index.html.erb

 <div>
-<h1>Conferences</h1>
+<h1 class="text-xl my-4">Conferences</h1>
 <ul>
 <% @conferences.each do |conference| %>
-<li><%= link_to conference.name, conference_path(conference.slug) %></li>
+<li><%= link_to conference.name, conference_path(conference.slug), class: "inline-block px-4 py-2" %></li>
 <% end %>
 </ul>
-</div>
+</div>
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