Allow specifying URL as input for custom sign in logo (#1330)

* Allow specifying URL as input for custom logos

* Fix typo

Co-authored-by: Joel Speed <Joel.speed@hotmail.co.uk>

* Update changelog

* Only allow HTTPS URLs

Co-authored-by: Joel Speed <Joel.speed@hotmail.co.uk>
Co-authored-by: Nick Meves <nicholas.meves@gmail.com>