[Cashtab] Run prettier on cashtab/src

Summary:
Have been seeing unrelated formatting changes creeping into diffs when files are touched. This is due to these files being formatted originally by the wrong version of prettier.

Run npm i -g prettier@2.6.0 and prettier src/ --write from cashtab/ dir to apply standard rules throughout repo.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16651