Implement largo_maybe_top_term

[benlk]

Changes

• Adds function largo_maybe_top_term( $args ), which outputs largo_top_term( $args ) wrapped in an h5.top-tag if largo_top_term( $args ) would return a link. If no link is returned, then it does not output the h5.top-term. $args are the same for each function.
• Adds a blank test for the function, so that we know what needs to be tested in it when we eventually write a test for it.

Why

• to reduce the number of conditionals in templates, and to finish fixing Top term possibly showing up twice on single posts? #1316 by using Should we wrap largo_top_term and create largo_maybe_top_term? #1318.
• to reduce number of database queries by halving the number of times we call largo_top_term

[aschweigert]

I'm ok with this for now, although the new default will/should be to just show the top term instead of doing some of this crazy conditional stuff. That's all very legacy at this point (a holdover from when people had the option of EITHER the top term or showing a list of tags below the excerpt in various rivers of stories). At this point, I think it makes sense to just show the top term by default in any of these contexts.

[benlk]

This is mainly for the cases where no top term is set, which is still a case that we need to worry about.