[alias-server] enforce alphanumeric aliases

Summary:
T2987

Add new function isValidAliasString which, for now, rejects any alias containing characters other than lower case a through z or numbers 0 through 9

Test Plan:
Review updates to mocks and npm test
Delete old test database and node index.js. Navigate to http://localhost:5000/aliases and no non-alphanumeric aliases

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D13313