High number of custom fields makes issue nav really slow in IE8

A SearcherModel is being created and added to the SearcherCollection for each custom field, on every search - even for custom fields hidden by context. This is causing JS execution times long enough in IE8 to bring up a "script running too long" error dialog. (Tested with 1000 custom fields)

A potential fix would be to skip the construction of SearcherModels for hidden custom fields, since they cannot be added to the search anyway. However, the More Criteria sparkler will need to be modified to still allow searching for those custom fields.

Also investigate additional strategies for improving performance when large numbers of custom fields are visible. Some easy wins:

• avoiding the use of _.bindAll in SearcherModel
• check if viewHtml is present before attempting to parse and clean it (viewHtml is not sent from the server if the field does not contain a value, but the code does unnecessary processing attempting to 'clean' this empty value anyway)