Make email change templates use main layout

Update the email change templates to fit with the rest of the main
site and use the standard notification template. Now they're far
prettier than before.

lms/templates/email_change_failed.html

-<h1>E-mail change failed.</h1>
+<%inherit file="main.html" />
 
-<p>We were unable to send a confirmation email to ${email}</p>
+<section class="container activation">
+
+  <section class="message">
+    <h1 class="invalid">E-mail change failed</h1>
+    <hr class="horizontal-divider">
+
+    <p>We were unable to send a confirmation email to ${email}</p>
+
+    <p>Go back to the <a href="/">home page</a>.</p>
+  </section>
+</section>

lms/templates/email_change_successful.html

-<h1>E-mail change successful!</h1> 
+<%! from django.core.urlresolvers import reverse %>
+<%inherit file="main.html" />
 
-<p>You should see your new email in your <a href="/dashboard">dashboard</a>.</p>
\ No newline at end of file
+<section class="container activation">
+
+  <section class="message">
+    <h1 class="valid">E-mail change successful!</h1>
+    <hr class="horizontal-divider">
+
+    <p>You should see your new email in your <a href="${reverse('dashboard')}">dashboard</a>.</p>
+  </section>
+</section>

lms/templates/email_exists.html

-<h1> Could not change e-mail </h1>
+<%inherit file="main.html" />
 
-An account with the new e-mail address already exists. Sorry. 
+<section class="container activation">
+
+  <section class="message">
+    <h1 class="invalid">E-mail change failed</h1>
+    <hr class="horizontal-divider">
+
+    <p>An account with the new e-mail address already exists.</p>
+
+    <p>Go back to the <a href="/">home page</a>.</p>
+  </section>
+</section>

lms/templates/emails_change_successful.html

-<h1>E-mail change successful!</h1> 
+<%! from django.core.urlresolvers import reverse %>
+<%inherit file="main.html" />
 
-<p> You should see your new name in your profile. 
+<section class="container activation">
+
+  <section class="message">
+    <h1 class="invalid">E-mail change successful!</h1>
+    <hr class="horizontal-divider">
+
+    <p>You should see your new email in your <a href="${reverse('dashboard')}">dashboard</a>.</p>
+  </section>
+</section>

lms/templates/invalid_email_key.html

-<h1>Invalid key</h1> 
+<%inherit file="main.html" />
 
-<p> This e-mail key is not valid. Please check: 
-<ul>
-<li> Was this key already used? Check whether the e-mail change has already happened. 
-<li> Did your e-mail client break the URL into two lines? 
-<li> The keys are valid for a limited amount of time. Has the key expired? 
-</ul>
+<section class="container activation">
+
+  <section class="message">
+    <h1 class="invalid">Invalid email change key</h1>
+    <hr class="horizontal-divider">
+    <p> This e-mail key is not valid. Please check:</p>
+    <ul>
+      <li>Was this key already used? Check whether the e-mail change has already happened.
+      <li>Did your e-mail client break the URL into two lines?
+      <li>The keys are valid for a limited amount of time. Has the key expired?
+    </ul>
+    <p>Go back to the <a href="/">home page</a>.</p>
+  </section>
+</section>