Make template tag `label` more robust.

This sort of code no longer throws an exception:

    <th>{{ lends|first|label:"foo"|capfirst }}</th>

If a non-existent label is requested, the template tag will return an empty
string.
Use template tag `label` more extensively.

There are now significantly fewer hard-coded column names in the templates. This
sort of thing:

    <th>Due out</th>

has been replaced by this sort of thing:

    <th>{{ lends|first|label:"due_out"|capfirst }}</th>