Handle relative links in auth method display icons

[gnprice]

In the get-server-settings response, each external auth method may have a display_icon field with a URL. We use those for showing an icon in the corresponding button on the login page.

Currently we interpret that URL string as an absolute URL. It's possible for it to be a relative URL string instead, though. That's long been accepted by the server from its settings, and has worked in the web app, and we're clarifying it in the API docs:

• https://github.com/zulip/zulip/pull/36459

We should start accepting that form too.

Implementation

A PR for this issue should include having manually tested it. That will probably require a test Zulip server.