[Cashtab] Improve SEO

Summary:
T2818

Add description and keyword meta tags to improve how search results are formatted.

Test Plan:
google.com and search Cashtab
Imagine it having "The official web wallet for eCash (XEC)" description instead of the current garbage

Reviewers: #bitcoin_abc, kieran709

Reviewed By: #bitcoin_abc, kieran709

Differential Revision: https://reviews.bitcoinabc.org/D12682