[explorer] testnet faucet page

Summary:
adding a page and some UI to interact with the testnet faucet

• page will be avaialbe on both explorers, but the link to it will only be visible on texplorer.e.cash
• users can enter an address in the input and it will display the api response

Test Plan:
preview the site and check /testnet-faucet
try entering a test address or an invalid address and check the response

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16890