Correct linting errors

Summary: Define default exports per updated lint specifications

Test Plan:

npm start

Observe no lint errors

Note: npm start is building broken css after some pkg upgrades from yesterday, to be corrected in later diff. Does not impact prod build.

Reviewers: alcipir, majcosta, deadalnix, #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8822