Plants taxonomic order another attempt

This has been a horrible change. I don't even have words to describe how badly I want this to go away from me.

-> the rebuild_taxonomy() procedure has been updated to include a separate calculation routine for the crazy plants
-> the Checklist::HigherTaxaInjector class has been modified so that it can be parametrized with "header ranks" (that default to the animalia set)
-> because the injector is now parametrised with kingdom, the injection step for pdf generation was moved to Checklist::Pdf::HistoryContent 
-> therefore, the Checklist::Pdf::HistoryFetcher could be removed

What I also wanted to do is change the way higher taxa injector works, so that instead of mixing Checklist::HigherTaxaItems and MTaxonConcepts in the json response we would have the higher taxa items in a separate array, with positions of taxon concepts where the headers need to go. That would be much cleaner. My intention was to use the same mechanism of returning higher taxa headers for the Species+ taxonomic results list. However, because this was taking too long and I was going crazy, I left this for now, hopefully will revisit when back to Species+.