Clearer HTML emails

Improve the presentation of information borne in HTML change emails by
reducing visual noise and emphasizing key elements. Together, the
following modifications establish this more-direct design:

* Salutations are written with a consistent voice across message types
  and draw attention to key words using bold.
* Change subjects are included in the salutation instead of extra
  paragraphs below.
* Removed the horizontal rule.
* The change link is made to be more prominent.
* Message footers are hidden in HTML emails. (Confirmed they they are
  still indexed for Gmail/Inbox search.)

Change-Id: Ia7b302b13764af2ab37de2bdb79b05cbb1d067c8

gerrit-server/src/main/resources/com/google/gerrit/server/mail/DeleteVoteHtml.soy

@@ -24,7 +24,8 @@
  */
 {template .DeleteVoteHtml autoescape="strict" kind="html"}
   <p>
-    {$fromName} removed a vote on this change.
+    <strong>{$fromName}</strong> removed a vote from{sp}
+    <strong>{$change.subject}</strong>.
   </p>
 
   {if $email.changeUrl}
@@ -33,11 +34,6 @@
     </p>
   {/if}
 
-  <p>
-    Change subject: {$change.subject}
-  </p>
-  <hr/>
-
   {if $coverLetter}
     <pre style="font-size:12px">{$coverLetter}</pre>
   {/if}