[alias-server] Move aliasConstants out of config and into constants dir

Summary:
T3060

With the spec solidified, these aren't really adjustable parameters. Will be permanent for prod version of app.

Move to /constants/ now to reflect this

Test Plan: npm test

Reviewers: #bitcoin_abc, emack, Fabien

Reviewed By: #bitcoin_abc, emack, Fabien

Subscribers: Fabien, emack

Differential Revision: https://reviews.bitcoinabc.org/D14046