[closes #155] make caldav url copyable on every browser

tracim/tracim/public/assets/css/dashboard.css

 .threads-history-reverse > i {
   font-size: 12px;
 }
+
+.user-edit-form__calendar__url {
+  padding: 8px 15px;
+  border: 1px solid #ccc;
+  border-radius: 4px;
+  background-color: #eee;
+  color: #555;
+}

tracim/tracim/templates/user_workspace_forms.mak

             <div class="form-group">
                 <label for="calendar">${_('Personal calendar')}</label>
                 <span class="info readonly">${_('This calendar URL will work with CalDav compatibles clients')}</span>
-                <input id="calendar" type="text" class="form-control"  disabled="disabled" value="${user.calendar_url}" />
+                <div class="user-edit-form__calendar__url">${user.calendar_url}</div>
             </div>
             <div class="form-group">
                 <label for="timezone">${_('Timezone')}</label>