Deck Browser: Scale clock and gear icons down

Patches DeckBrowser#_deckRow() and DeckBrowser#_dueImg() to correctly
scale icons down to fit into the HTML region.
Custom Study Tag Dialog: Add missing image note

TagLimit#rebuildTagList() refers to a nonexistent image (Anki_Fact.png)
which does not exist. This possibly intends to refer to anki-tag.png
instead.