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[bot]]

Codecov Report

Merging #4281 (542bc2e) into master (144b8aa) will increase coverage by 0.08%. The diff coverage is 85.13%.

@@            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              

Impacted Files	Coverage Δ
src/components/BusyBottom/constants.ts	100.00% <ø> (ø)
src/components/SignTransactionWallets/index.ts	100.00% <ø> (ø)
src/config/wallets.ts	100.00% <ø> (ø)
src/features/AddAccount/AddAccountForm.reducer.tsx	15.00% <0.00%> (-0.39%)	:arrow_down:
src/features/AddAccount/stories.tsx	100.00% <ø> (ø)
src/features/BroadcastTransaction/helpers.ts	0.00% <0.00%> (ø)
src/index.tsx	0.00% <ø> (ø)
...es/BroadcastTransaction/components/BroadcastTx.tsx	89.28% <72.22%> (-5.96%)	:arrow_down:
src/components/SignTransactionWallets/Offline.tsx	100.00% <100.00%> (ø)
src/components/WalletUnlock/Offline.tsx	100.00% <100.00%> (ø)
... and 8 more

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