Add offline account functionality to use with Quill

[Mrtenz]

This PR adds a new account type called "Offline account", which can be used together with Quill to sign transactions offline.

[mycrypto-bot]

Staging build: MyCryptoBuilds

[codecov]

Codecov Report

Attention: Patch coverage is 85.13514% with 11 lines in your changes missing coverage. Please review.

Project coverage is 78.82%. Comparing base (144b8aa) to head (542bc2e).
Report is 10 commits behind head on master.

Files with missing lines	Patch %	Lines
...es/BroadcastTransaction/components/BroadcastTx.tsx	72.22%	5 Missing ⚠️
src/features/BroadcastTransaction/helpers.ts	0.00%	5 Missing ⚠️
src/features/AddAccount/AddAccountForm.reducer.tsx	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4281      +/-   ##
==========================================
+ Coverage   78.73%   78.82%   +0.08%     
==========================================
  Files         572      576       +4     
  Lines       12429    12498      +69     
  Branches     3068     3085      +17     
==========================================
+ Hits         9786     9851      +65     
- Misses       2611     2615       +4     
  Partials       32       32              

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