Serve static files and add iconUrl property to sensor

[mpfeil]

Description

Add a basic /assets route that serves static files

Motivation and Context

See issue #117

How Has This Been Tested?

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [ ] My code has been linted using yarn run lint.
• [ ] My code does not break the tests (yarn run test)
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.