Update Coinbase icons

[tjorim]

Description:

This PR adds extra icons (EUR, LTC, USD) and puts them in a dictionary.
Furthermore, it doesn't rely on the name anymore but the actual currency.
We get the icon from the dict with a safe get() and default value as before.

Related issue (if applicable): fixes # NA

Pull request in home-assistant.io with documentation (if applicable): home-assistant/home-assistant.io#<home-assistant.io PR number goes here> NA

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in home-assistant.io

If the code communicates with devices, web services, or third-party tools:

• New dependencies have been added to the REQUIREMENTS variable (example).
• New dependencies are only imported inside functions that use them (example).
• New or updated dependencies have been added to requirements_all.txt by running script/gen_requirements_all.py.
• New files were added to .coveragerc.

If the code does not interact with devices:

• Tests have been added to verify that the new code works.

[fabaff]

Looks good to me 🐦