Add flow support to relative paths

[davet2001]

Description

#6411 added relative paths to allow the web root for mitmproxy to be placed in locations other than '/'.

But some items were missed from this change, especially manipulating and saving flows, see #6525
This PR modifies several js components so that POST API features work even when the web root is not '/'.

Checklist

• I have updated tests where applicable.
• I have added an entry to the CHANGELOG.

[davet2001]

Ok, this is partially working. I don't think I can get much further without making the js compile locally and running the full test suite.
@mhils are there any instructions on how to set this up? Also is there any way of running them in a devcontainer?

[mhils]

You can compile the JS with npm start prod: https://github.com/mitmproxy/mitmproxy/blob/main/release/release.py#L79-L83 :)

[davet2001]

I am a bit baffled by the test results. Unfortunately am not at all proficient at npm. I was able to reproduce the npm test failure locally, but can't figure out why it is receiving a completely different POST command to what the test expects e.g.:

    Expected: "/flows/d91165be-ca1f-4612-88a9-c0f8696f3e29/kill", {"method": "POST"}
    Received
           1
              "flows/d91165be-ca1f-4612-88a9-c0f8696f3e29/resume",
              {"method": "POST"},
           2
              "flows/resume",
              {"method": "POST"},
           3
              "flows/d91165be-ca1f-4612-88a9-c0f8696f3e29/kill",
              {"method": "POST"},

Have I messed something up with the JS compilation maybe? Please help :)

[mhils]

Sorry for the slow RTT. I was trying to get this done today, but there's a bunch of stuff that isn't mergable at the moment. The errors you are seeing are caused by test failures, check out the mentioned tests and things should become relative clear. Thanks!

[mhils]

Calls to fetchApi should still be prefixed with /, fetchApi has explicit logic to make them relative.

[mhils]

This breaks all existing links to mitmweb instances, which I'd like to avoid. location.hash does not need relative URLs.