gateway: make it easier to retry from HTML error page for 504 Timeouts

[lidel]

Problem

When we timeout, gateway shows non-actionable error that says "context deadline expired".

The text/html response returned to browser users is not much better, just prettier.

Very often refreshing page again will succeed, but users do not know that.

Solution

We should make text/html responses for HTTP 504 errors actionable.

Initial ideas

• (MVP) Add "Retry" button that forces 504 error page to reload (window.location.reload()?)
• (optional/additional) Add <meta http-equiv="refresh" content="60"/> which will automatically refresh the tab after 1 minute (this way if someone left page loading, and it errored, it may self-heal before user is back to notice the error)