[examiner] Adds permission check when adding examiner to site.

[racostas]

Brief summary of changes

Permissions are check now before allowing to add examiner to site.
Before this PR, a user with only the permission Examiner: Add and Certify Examiners - Own Sites was able to add examiners to sites it don't belongs to. Now this feature is only granted to users with the corresponding level of permission.

Testing instructions (if applicable)

1. With a user that only have the permission Examiner: Add and Certify Examiners - Own Sites try to create a new examiner in MainMenu->Clinical->Examinier
2. When clicking the Add Examiner button the modal windows that opened should only list the sites the current user belongs to.
3. See image as reference.
   
4. Only for the listed sites the current user should be able to add an examiner.

Link(s) to related issue(s)

• Resolves [examiner] I was able to create an examiner for a site I don't belong to #9149

[sruthymathew123]

@driusan Tested and looks good on my end.