use proper URL base for Search#all_courses API pagination
use proper URL base for Search#all_courses API pagination

fixes CNVS-30621

test plan:
 * visit /api/v1/search/all_courses
 * the link header should contain protocol, hostname, and api/v1

Change-Id: I3736101cfd19a38d2d87d78cd246caac327f49af
Reviewed-on: https://gerrit.instructure.com/86831
Reviewed-by: Simon Williams <088e16a1019277b15d58faf0541e11910eb756f6@instructure.com>
Tested-by: Jenkins
QA-Review: Jeremy Putnam <b030d6d6f199f844ec424882be473c2ab9adc93b@instructure.com>
Product-Review: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>

HotChalk/canvas-lms,SwinburneOnline/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,venturehive/canvas-lms,sfu/canvas-lms,SwinburneOnline/canvas-lms,instructure/canvas-lms,grahamb/canvas-lms,matematikk-mooc/canvas-lms,instructure/canvas-lms,dgynn/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,HotChalk/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,venturehive/canvas-lms,dgynn/canvas-lms,HotChalk/canvas-lms,grahamb/canvas-lms,instructure/canvas-lms,venturehive/canvas-lms,SwinburneOnline/canvas-lms,djbender/canvas-lms,djbender/canvas-lms,dgynn/canvas-lms,matematikk-mooc/canvas-lms,sfu/canvas-lms,dgynn/canvas-lms,matematikk-mooc/canvas-lms,matematikk-mooc/canvas-lms,venturehive/canvas-lms,djbender/canvas-lms,grahamb/canvas-lms,instructure/canvas-lms,HotChalk/canvas-lms,grahamb/canvas-lms,SwinburneOnline/canvas-lms,grahamb/canvas-lms