Inform user about limited capacity of the cart

[fedorov]

When measured in the number of items added by clicking on individual items, IDC cart has limited capacity.

To address this we should:

1. Next to the cart add some kind of a UI thing that will inform the user about this limit
2. When use approaches the limit while filling the cart, they should also get a popup warning; once the limit is hit, do not allow clicking on the manifest button until cart is at or below capacity.

[fedorov]

Assigning to @G-White-ISB who appears to have cycles, according to @wlongabaugh

[wlongabaugh]

Further discussions yesterday indicated a preference to show a running count in the cart panel, e.g.:

Cart contents: 6690 series from 1 collections / 20 cases / 589 studies (20/1000 allowed selections)

[wlongabaugh]

Plus, the manifest button would be disabled as long as the number of selections exceeds the limit.