Don't hard code style; use class and improve CSS

This changes the hard-coded style for what I'm calling the
content-with-donate-sidebar.  The advantage of not hard-coding style are
obvious, but I'm doing this now rather than later so that I can add
changes to the CSS that causes the width to extend to 100% on smaller
screen media when the donate bar disappears (the latter of which is
already implemented).

www/conservancy/static/conservancy.css

@@ -36,6 +36,10 @@ text-decoration: none; color: #557733;
 }
 @media all and (max-width: 625px) {
     .donate-sidebar { display: none; }
+    .content-with-donate-sidebar {
+        align: center;
+        width: 100%;
+    }
 }
 
 p.footnote {
@@ -113,6 +117,11 @@ h3 { margin-top: .6em; margin-bottom: .4em; }
     margin-right: 50px;
 }
 
+.content-with-donate-sidebar {
+    align: left;
+    width: 80%;
+}
+
 #container {
     width: 100%;
     overflow: hidden;

www/conservancy/static/npoacct/index.html

@@ -268,7 +268,7 @@ Other donation methods:
 </td></tr></table>
 </div>
 
-<div style="align:left;width:80%;">
+<div class="content-with-donate-sidebar">
 <!-- begin campaign text -->
 
 <h2>Fundraising Campaign: Non-Profit Accounting Software</h2>

www/conservancy/static/supporter/event.html

@@ -12,7 +12,7 @@
 {% endblock %}
 
 {% block content %}
-<div style="align:left;width:80%;">
+<div class="content-with-donate-sidebar">
   <h1>Conservancy Supporter Night 2015</h1>
   <p>
   <dl>

www/conservancy/static/supporter/index.html

@@ -55,7 +55,7 @@ Other annual supporters methods:
 </div>
 
 
-<div style="align:left;width:80%;">
+<div class="content-with-donate-sidebar">
   <h1><img class="appeal-header" alt="Become a Conservancy Supporter!" src="/img/conservancy-supporter-header.png"/></h1>
 
 <p>Software Freedom Conservancy is an essential organization to free and

www/conservancy/templates/base_compliance.html

@@ -38,7 +38,7 @@ of $<span id="fundraiser-goal">50,000</span> match met.<br/>
       Supporter</strong></a>.</p>
 </td></tr></table>
 </div>
-<div style="align:left;width:80%;">
+<div class="content-with-donate-sidebar">
     <div id="container">
          <div id="sidebar" class="{% block submenuselection %}other{% endblock %}">
             <h2>Our GPL {% block category %}Compliance{% endblock %} Work</h2>