Lookup namespace by pages host using find_by_path

What does this MR do?

In Namespace#find_by_pages_host we always have top level namespaces which means we can use find_by_path instead of find_by_full_path and avoid join on routes.

Some benchmarks done locally with benchmark/ips

Calculating -------------------------------------
Namespace.find_by_full_path
                        31.000  i/100ms
Namespace.find_by_path
                       107.000  i/100ms
-------------------------------------------------
Namespace.find_by_full_path
                        331.125  (±16.9%) i/s -      1.612k
Namespace.find_by_path
                          1.179k (±20.3%) i/s -      5.671k

Comparison:
Namespace.find_by_path:     1178.9 i/s
Namespace.find_by_full_path:      331.1 i/s - 3.56x slower

Related to #207096 (closed).

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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• Label as security and @ mention @gitlab-com/gl-security/appsec
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