Dispatch AVCaptureDevice.requestAccess completion to the main queue

[mattrubin]

This fixes a bug where the "needs permission" message would not appear correctly on the camera view immediately after the user denied access to the camera.

From the AVCaptureDevice.requestAccess documentation:

The completion handler is called on an arbitrary dispatch queue. It is the client's responsibility to ensure that any UIKit-related updates are called on the main queue or main thread as a result.

[codecov]

Codecov Report

Merging #293 into develop will decrease coverage by 0.04%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           develop    #293      +/-   ##
==========================================
- Coverage    38.34%   38.3%   -0.05%     
==========================================
  Files           40      40              
  Lines         1870    1872       +2     
==========================================
  Hits           717     717              
- Misses        1153    1155       +2

Impacted Files	Coverage Δ
Authenticator/Source/QRScanner.swift	3.92% <0%> (-0.17%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8cc2dc7...5ad284c. Read the comment docs.