Asks user for confirmation before deleting token from device

[beaucollins]

Uses UIAlertController to present a UIActionSheet style dialog asking the user if they
want to remove the token.

Fixes #198

[beaucollins]

@mattrubin this should help prevent any accidental deletions.

I think it could be interesting to explore making an "undo" user flow possible versus asking for confirmation.

[codecov]

Codecov Report

Merging #217 into develop will decrease coverage by 0.43%.
The diff coverage is 0%.

@@             Coverage Diff             @@
##           develop     #217      +/-   ##
===========================================
- Coverage    40.64%   40.21%   -0.44%     
===========================================
  Files           36       36              
  Lines         2244     2268      +24     
===========================================
  Hits           912      912              
- Misses        1332     1356      +24

Impacted Files	Coverage Δ
Authenticator/Source/AppController.swift	22.92% <0%> (-4.14%)	⬇️

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[mattrubin]

I've updated this PR to tweak the confirmation message text, and to present the confirmation in the more prominent alert style.