taxon: Use strikethrough when displaying invalid names

[synrg]

Sometimes a search matches an invalid name (scientific or otherwise; e.g. Lymantria dispar's old common name, and some others that are now considered names that should no longer be used are marked invalid). When that happens, to make it clear that the name is invalid, use strikethrough style.

This isn't just a straightforward application of style, because first we need to determine if the matching term is invalid:

• first, the matched_term needs to be checked against all names on the full taxon record
  • these need to be retrieved with all_names=true (but we do this already to support other features)
• then determine if the matched_term exactly matches an invalid name (i.e. one that has is_valid == false)
• then apply the strikethrough style if it does

[synrg]

Implemented in 3908f37a2908aa0044aa2b00eaee0096b3306f87