dashboard: a Slipstream page

[rsantacroce]

What

This is the dashboard half of slipstream, stacked on #83. Its base is feat/slipstream-service, so the diff here is only the dashboard. It retargets to main once #83 merges.

A public /slipstream page, shown in the nav only when SLIPSTREAM_API_URL is set. It has four parts:

• Fees: the minimum submission rate, the current mineable rate, what is required now, and how recently that was read from the template.
• Submit: a curl line pointing at PUBLIC_SLIPSTREAM_URL, and a link to the pool's info.json.
• Recent transactions: each with its fee rate and status.
  • Statuses: in template, mined, confirmed, or dropped with the node's reason.
  • The block is shown with (ours) or (another pool). Accepted txs are broadcast, so another pool can mine one first, and the page says so.

Design notes

• The page reads the slipstream service's API, not slipstream.db. The service stays the only owner of its schema.
• If the service is down, the page says so. It costs the page one card, not the page, and the rest of the dashboard is unaffected.
• There is no admin withdraw action. An earlier version of this PR had one, which removed the tx from an enforcer-side slipstream pool. slipstream: take txs from anyone, get them into the pool's blocks #83 now submits to the pool's bitcoind instead, and Core has no RPC to take a tx back out of its mempool. The admin router and the Tools page are unchanged from main.

Testing

• dashboard/test/slipstream.test.js (4 tests):
  • The page renders both rates, the submit line, statuses and reasons.
  • It degrades when the service is down.
  • The nav link appears only when slipstream is enabled.
  • fetchSlipstream works against a stub service.
• The full dashboard suite passes: 193 tests.
• Live, against the regtest stack (an earlier revision, with the service's former backend): the page rendered a submitted tx through to confirmed at its block height. The page only reads the service's API, and the API's shape did not change with the backend.

Found, not fixed here

Without a shares.db (a proxy that has never run), / returns 500 from views/index.ejs:71. This is the same on the base branch, so it is not from this change.