General UI improvements

[JWCook]

Updates #203

Lots of misc improvements:

• Use pyqtdarktheme instead of qtmodern, and fix custom stylesheet and palette to be compatible
• Fix infocard title text stealing mouse focus on click events
• Disable main taxon/observation photo hover and click events until first photo is loaded
• Fix cutoff issue for small icons
• Fix padding of metadata icons
• Fix alignment/padding for taxon rank dropdowns
• Draw rounded corners on thumbnails
• Fix casing for common names with apostrophes on TaxonInfoCards
• Minor fix for null identifications