[alias-server] add cors support

Summary:
T3001

Depends on D13346

Add cors support so Cashtab + other websites can use the server

Test Plan:
tbh I'm not sure how to test this locally. I've used this approach on other servers. We can also expand this to whitelist certain domains if we are getting traffic problems.
node index.js and make sure you can still navigate to localhost:5000/aliases

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D13347