get rid of email card border

src/metabase/email/alert_admin_unsubscribed_you.mustache

 {{> metabase/email/_header }}
-  <div style="padding: 2em 4em; border: 1px solid #dddddd; border-radius: 2px; background-color: white; box-shadow: 0 1px 2px rgba(0, 0, 0, .08);">
     <p>Just letting you know that {{adminName}} unsubscribed you from alerts about <a href="{{questionURL}}">{{questionName}}</a>.</p>
-  </div>
 {{> metabase/email/_footer}}

src/metabase/email/alert_new_confirmation.mustache

 {{> metabase/email/_header }}
-  <div style="padding: 2em 4em; border: 1px solid #dddddd; border-radius: 2px; background-color: white; box-shadow: 0 1px 2px rgba(0, 0, 0, .08);">
     <p>This is just a confirmation that your alert about <a href="{{questionURL}}">{{questionName}}</a> has been set up.</p>
     <p>This alert will be sent {{alertCondition}}.</p>
-  </div>
 {{> metabase/email/_footer}}

src/metabase/email/alert_stopped_working.mustache

 {{> metabase/email/_header }}
-  <div style="padding: 2em 4em; border: 1px solid #dddddd; border-radius: 2px; background-color: white; box-shadow: 0 1px 2px rgba(0, 0, 0, .08);">
-    <p>Alerts about {{questionName}} have stopped because {{deletionCause}}.
-  </div>
+    <p>Alerts about {{questionName}} have stopped because {{deletionCause}}.</p>
 {{> metabase/email/_footer}}

src/metabase/email/alert_unsubscribed.mustache

 {{> metabase/email/_header }}
-  <div style="padding: 2em 4em; border: 1px solid #dddddd; border-radius: 2px; background-color: white; box-shadow: 0 1px 2px rgba(0, 0, 0, .08);">
     <p>You’re no longer receiving alerts about <a href="{{questionURL}}">{{questionName}}</a>.</p>
-  </div>
 {{> metabase/email/_footer}}

src/metabase/email/alert_was_deleted.mustache

 {{> metabase/email/_header }}
-  <div style="padding: 2em 4em; border: 1px solid #dddddd; border-radius: 2px; background-color: white; box-shadow: 0 1px 2px rgba(0, 0, 0, .08);">
     <p>{{adminName}} deleted an alert you created about <a href="{{questionURL}}">{{questionName}}.</p>
     <p>Since we're in the business of alerts, we thought we'd alert you.</p>
-  </div>
 {{> metabase/email/_footer}}

src/metabase/email/alert_you_were_added.mustache

 {{> metabase/email/_header }}
-  <div style="padding: 2em 4em; border: 1px solid #dddddd; border-radius: 2px; background-color: white; box-shadow: 0 1px 2px rgba(0, 0, 0, .08);">
     <p>You’re now getting alerts about <a href="{{questionURL}}">{{questionName}}</a>.</p>
-    <p>We’ll send you an email {{alertCondition}}</p>
+    <p>We’ll send you an email {{alertCondition}}.</p>
     <p>If you don’t want to receive these alerts, you can <a href="{{questionURL}}">go to this question</a> and unsubscribe from the menu in the top-right.</p>
-  </div>
 {{> metabase/email/_footer}}