query: Fix external prefix and add an e2e test for it

[onprem]

• I added CHANGELOG entry for this change.
• Change is not relevant to the end user.

Changes

• Test --web.external-prefix using a simple reverse proxy
• Test both --web.external-prefix and --web.route-prefix in a single scenario, i.e. when both are set simultaneously.

Changes in externalPrefix and routePrefix handling

• Now, --web.route-prefix defaults to --web.external-prefix. This behavior allows a user to use thanos with and without a reverse proxy if --web.external-prefix is set, and it is in line with what Prometheus does.
• Updated both, the old UI and the new React UI to account for --web.external-prefix

Turns out, --web.external-prefix is still broken, and as expected, these tests would fail unless we fix it.
It should work now.

This PR changes how --web.external-prefix works, so it can make the UI inaccessible to some users who were relying on some workarounds or hacks before (like setting --web.external-prefix=".").

Verification

Ran GOTEST_OPTS="-run TestQuery" make test-e2e locally to test all e2e tests related to Query component.

[onprem]

Now that we are more in line with what Prometheus does, the following guides also works for Thanos.

• https://www.robustperception.io/using-external-urls-and-proxies-with-prometheus
• https://www.robustperception.io/external-urls-and-path-prefixes

The only difference is that, instead of using the Prometheus flag --web.external-url one needs to use --web.external-prefix flag in Thanos. So prometheus --web.external-url http://localhost:19090/prometheus/ becomes thanos query --web.external-prefix=thanos for Thanos.

[GiedriusS]

Awesome work! Just a few comments on how to improve this 🎉

[onprem]

Seems like Netlify is having some problems.

[GiedriusS]

The code looks good! I think we should another iteration on the documentation and then we can merge this :P Mostly grammar nits + renamed Thanos querier to Thanos Query to be consistent.

[GiedriusS]

LGTM. Even tested with a reverse proxy with the example configuration in the documentation and it works with these changes whereas with the master version you get redirected to http://localhost/thanos/thanos/graph with identical configuration.
I'll give @squat and others time to review these changes if they want to before merging this. Awesome stuff!

[squat]

Looks great overall, Prem! Great work and thanks for the docs! Just some small nits

[squat]

Great 🎉

[GiedriusS]

Oops, fixing up the CHANGELOG in #2833.