tagged-entry: Use the entry-tag CSS style class from Adwaita

The dark and light variants of the CSS for GdTaggedEntryTag were still
scattered across applications and libgd as the photos-entry-tag and
documents-entry-tag style classes. It has been decided to consolidate
all that in Adwaita, like we do for the other libgd style classes, as
the "entry-tag" style class.

https://bugzilla.gnome.org/show_bug.cgi?id=781671