[e.cash] Consolidate redirect definitions

Summary:
Currently site redirects are defined in each config file (one is for prod/dev and one is for the preview bot)
Moving these to a shared location for easier maintenance

Test Plan:
Bot preview the site and check all the redirects are working
Can also run locally and add a random redirect to check functionality

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16357