[HttpKernel] changed fragment URLs to be relative by default (closes #8458)
merged branch fabpot/fragment-urls (PR #8879)

This PR was merged into the 2.2 branch.

Discussion
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Made fragment URLs relative instead of absolute by default

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #8458
| License       | MIT
| Doc PR        | n/a

This fixes a regression introduced with the new fragment system where fragment URLs were generated as absolute URLs. As per the ESI spec, there is no need to generate an absolute URL and this can even be problematic when dealing with internal sub-requests in a more "complex" hosting environment.

Commits
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91234cd [HttpKernel] changed fragment URLs to be relative by default (closes #8458)