Improve button accessibility

[mattrubin]

Add accessibility labels and hints to custom buttons. Closes #190.

There are further accessibility refinements that can be made, but these changes begin by making all elements in the app compatible with VoiceOver.

[codecov]

Codecov Report

Merging #207 into develop will increase coverage by 0.05%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop     #207      +/-   ##
===========================================
+ Coverage    37.72%   37.78%   +0.05%     
===========================================
  Files           36       36              
  Lines         2139     2141       +2     
===========================================
+ Hits           807      809       +2     
  Misses        1332     1332

Impacted Files	Coverage Δ
...henticator/Source/TokenScannerViewController.swift	0% <ø> (ø)	⬆️
Authenticator/Source/TokenListViewController.swift	64.9% <ø> (ø)	⬆️
Authenticator/Source/TokenRowCell.swift	85.71% <100%> (+0.34%)	⬆️

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