Use tx service for loading token info

[tschubotz]

What is this feature about? (1 sentence)

This will use the Safe transaction service for pulling token info instead of the Safe relay service.

Why is it needed? What is the value? For whom do we build it?

• There is additional effort to run the relay service. Hence we made the decision to have token info on the tx service moving forward.
• On xDai and EWC/Volta we don't have the relay service. (xDai compatibility #1353, EWC & Volta compatibility #1253)
• @rmeissner might be able to add more "why?"

Task

• Load token info from the tx service for all networks (Mainnet, Rinkeby, xDai, ...)
  • https://safe-transaction.gnosis.io/api/v1/tokens/
  • https://safe-transaction.rinkeby.gnosis.io/api/v1/tokens/
  • https://safe-transaction.xdai.gnosis.io/api/v1/tokens/

[tschubotz]

Moving this to high prio since it's needed for #1353 and #1253