PR : correction for Bing search image api endpoint

[PooLP]

Q	A
Bug fix?	[x ]
New feature?	[ ]
New sample?	[ ]
Related issues?	fixes #1256

What's in this Pull Request?

fix(FilePicker) : correction for Bing search image api endpoint
doc(FilePicker) : add precision for Bing Search image API

[PooLP]

I have modified "toRelativeUrl" method because if the regEx is called at the root site level causes performance problem with long url.
Test this on Chrome console to test performance problem :
"https://superlongspnamewithmoreword.sharepoint.com".replace(/^(?:\/\/|[^/]+)*\//, '/');

[PooLP]

I have modified the Url build for getListItems method on OrgAssetsService because the old builder didn't work if file picker is used with organisation asset/lib declared on root site.

The modification is not perfect, but I didn't want to add side effects by modifying the "buildAbsoluteUrl" method.

The test requires having 2 test environments, 1 with org assets declared in root side, a secondary with org assets declared in another site collection. you can't have both at the same time/tenant.

[AJIXuMuK]

Thank you @PooLP for the fix!