[Cashtab] fix css issue with long wallet display names

Summary: Reduce font size for active wallet name at resolutions where it would overflow. Truncate wallet name in Saved Wallets list if it overflows its divs. Ellipsis truncation approach for SavedWallets lists deemed more robust than highly specific font size resolutions by screen size; displaying the last few characters of a long wallet name is not critical.

Test Plan:

cd web/cashtab
npm start

Create a new wallet and rename it with longest possible name, e.g. AAAAAAAAAAAAAAAAAAAAAAA

Using dev console of web browser, test appearance of wallet name at resolutions between 320px and 1280px. Verify either no overflow or overflow handled by ellipsis.

npm test to verify no impact on covered methods; change only impacts app appearance.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9243