[Cashtab] Close window after a token send tx from link

Summary: This is current behavior for link txs that send xec. Should also apply for token txs. Oversight that this is not in yet.

Test Plan: npm test, no specific test for this, send a token tx and note that the tab (or extension) closes.

Reviewers: #bitcoin_abc, johnkuney

Reviewed By: #bitcoin_abc, johnkuney

Differential Revision: https://reviews.bitcoinabc.org/D18134