Sorting of box/folder and other fields that may be integers

[cbeer]

Transferring projectblacklight/spotlight#1445

In some exhibits (such as Feigenbaum), there are fields (like box and folder) that only have integer values. Because these fields may hold non-numeric values in other exhibits, we use string field types to store in solr. This leads to odd sorting behavior when the user selects A-Z sort, e.g.

1
10
11
12
13
14
15
16
17
18
19
2
20
etc.

Consider alternatives to how this can work better for exhibits that really treat these fields as numbers to improve the user experience (for example, copy fields into alternate numeric solr fields that can be optionally shown for only those exhibits).