Fixed #21460 -- Reenabled proper template precedence in find_template

Refs #20806. Thanks Unai Zalakain for the review.
Fixed missing admindocs' site_header
Merge pull request #1877 from Bouke/patches/4

Fixed missing admindocs' site_header, refs #21293
Fixed #20867 -- Added the Form.add_error() method.

Refs #20199 #16986.

Thanks @akaariai, @bmispelon, @mjtamlyn, @timgraham for the reviews.
Removed Form._errors from the docs in favor of the add_error API.