[Cashtab] Validation function for new wallet name length

Summary: Added function + tests in validation.js & validation.test.js that handles checking new wallet name length. Related to T2409.

Test Plan:
cd web/cashtab && npm start
navigate to configure screen
ensure there is at least one wallet in saved wallets list
click rename wallet button
enter a wallet name longer than 24 characters
observe that the error message for a wallet name that is too long will appear when max character limit is exceeded
backspace the wallet name until there are 0 characters left in the input
observe that the error message displays when the wallet name is 0 characters long
enter a valid wallet name to the input
observe that the wallet name successfully updates on Ok.

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D11835