Configuration options for term display

Some sites will need to limit the number of feeds displayed for terms when vocabularies are large. As noted in code comments, the current approach is limited:

// Only show first 30 terms. Wary of huge vocabs. Not ideal.

The attached patch introduces two new configuration options.

The first setting allows a site admin to set a maximum term depth. When set, terms in all nested vocabularies will display only to that depth (e.g., 2), defaulting to no limit.

The second setting is a configurable limit on the number of terms per vocabulary, defaulting to the current limit of 30.

Together, these configuration options give site admins better control over term feed display.