Icon Sprite URL needs to be local even if asset_host is set

See merge request gitlab-org/gitlab-ce!15289

(cherry picked from commit e482a4d5)

25eea058 Icon Sprite URL is also local even if asset_host is set
81eaf171 Fixed URL Matching in Tests
687e7dea Put Icons Path in Helper
fb1b88de Missing the local url
01240a18 Fixed Function + Test based on MR
fce34b4b Removed Brackets