Fix for Bug 6484 - due date sorter on 'my summary' missing

Fix for Bug 6484 - due date sorter on 'my summary' missing

Columns on the summary table are shown or hidden based on a
variety of different settings, leading to convoluted if/else
template constructions if we want to define header sorting
in the <script> block. Using the jQuery Metadata plugin
allows us to specify sorting options as classes on the
table header. This adds a dependency but simplifies the
template markup significantly.

I recommend continuing to specify header sort options in the
<script> block in cases where it is relatively simple to do
so and saving inclusion of the additional jQuery plugin for
pages which really need it.

Other changes: Moved common sorter configuration detalis to
default declaration; Moved the date due column so that it
could be set as the default sort without taking into account
multiple disappearing columns (default sort cannot be set
in the markup for a <th>).

Signed-off-by: Nicole C. Engard <nengard@bywatersolutions.com>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>