Use icons from mempool (#347)

* change script to get icons

* change icons to data:image/png;base64 from blockstream registry

* prettier

* use icons from mempool

* fetch mempool supported hashes from github

* prettier

* Removes:
  - scripts/pull-asset-registry.js
  - src/application/constants/blockstream_asset_hash.json
  - src/application/constants/lightnite_asset_hash.json
  - public/assets/images/liquid-assets

Implements new asset icon logic:
  - serve icon from https://liquid.network/api/v1/asset/<hash>/icon
  - on error change img src to /assets/images/question-mark.svg
  - use mainnet hash asset when dealing with featured assets on different network

Featured assets:
  - lbtc
  - lcad
  - usdt

* featured asset icons are now served locally