[e.cash] Rename CustomLink to ExternalLink

Summary: This name is a better description of what it is

Test Plan:
git grep CustomLink
Run the site locally, click on various links to check that they still behave as expected

Reviewers: johnkuney, #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14763