Adding the explorer to the monorepo

Summary: As requested, adding the explorer.e.cash repo to the mono repo

Test Plan: Open the explorer folder in the web directory. Try running the app (see the readme for instructions)

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Mengerian, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12889