Update tag validation

Fixes #536

static/directives/tag-operations-dialog.html

@@ -34,7 +34,7 @@
             <div ng-show="!addingTag">
               <input type="text" class="form-control" id="tagName"
                      placeholder="Enter tag name"
-                     ng-model="tagToCreate" ng-pattern="/^([a-z0-9_\.-]){3,30}$/"
+                     ng-model="tagToCreate" ng-pattern="/^[\w][\w\.-]{0,127}$/"
                      ng-disabled="creatingTag" autofocus required>
 
               <div style="margin: 10px; margin-top: 20px;"
@@ -121,4 +121,4 @@
     <span class="label label-default tag">{{ revertTagInfo.tag.name }}</span> to image
     <span class="image-id">{{ revertTagInfo.image_id.substr(0, 12) }}?</span>
   </div>
-</div>
+</div>