Try fixing custom network related cors issues

[donfrigo]

Description of the Change

Found a possible fix on Stackoverflow and on the web3 library github page, so in this PR the web3 library is rolled back to 0.20.6.

Issues

Relates to #55. Needs verifications.

[filippsen]

The linked reports relate to a different error (Access-Control-Allow-Origin), so it is unclear how this change is going to solve the problem with CORS settings.

[test]

• Compiles from scratch.

editor: Compiling...
editor: Compiled successfully!

OK

• Double checking dapps after the package update:
  • HelloWorld: Compile, Deploy to Browser, Preview and Interact are apparently working as intended. Deploy to Custom, OK.
  • CryptoPizza: Compile, Deploy to Browser, Preview and Interact OK

[notes]

• Needs to be verified live;
• If that indeed resolves the CORS settings and remote resource access, issue Configure Account returns could not get balance for unlocked custom accounts #55 is expected to be solved;
• If this does not bring any valuable difference, consider rolling back this request as newer library version is to be favored.