[e.cash] update MineMine brand to letsmineit

Summary:
updating minemine to letsmineit and updating links as follows:
https://letsmine.it/pool/XECPOOL (pool)
https://letsmine.it/coin/xec (solo)

Test Plan: visit /mining under get-ecash and find "letsmineit" to check links. Check that any reference of "minemine" is removed.

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D18512