Couple of bugfixes for canonical url

• Fix canonical.base_url property name
• Use homepage setting without explicitly specifying the scope

https://github.com/lightbend/paradox/milestone/17?closed=1

Author	Commits	Lines added	Lines removed
2m	3	14	2
ennru	2	2	2
rstento	1	2	2
scala-steward	1	1	1