Add pages unique domain option

What does this MR do and why?

Problem

Currently, cookies of different gitlab-pages sites under the same namespace are leaked due to the gitlab-pages default URL format: namespace.gitlab.io/project

Solution

Add an option to enable users to create a unique subdomain for a gitlab-pages project, like: project-namespace-somerandomvalue.gitlab.io

How

based on the PoC !106961 (closed)

1. Create two columns in the project_settings table:
   • pages_unique_domain (text): where the unique domain will be stored. (limit of 63 chars - https://www.freesoft.org/CIE/RFC/1035/9.htm)
   • pages_unique_domain_enabled (bool): if the unique domain feature is enabled

This way, we can keep the unique domain even if the user disables the feature.

1. Add a checkbox in namespace/project/pages (Project pages settings) allowing the user to enable the feature (pages_unique_domain_enabled)
   • Use a feature flag to control the checkbox
2. The unique subdomain can follow a format like: projectpath-namespacepath-randomstring
   • project path: up to 48 chars
   • namespace path: complete value up to 48 chars
   • random string: SecureRandom.hex(size) to fill the value to 63 chars

Examples:

project path	namespace path	unique domain
project	namespace	project-namespace-c226671dcc75af60ff0c42aa4b46cdc4704d3b83c302a
pppppppppppppppppppppppppppppppppppppppp (40 chars)	namespace	pppppppppppppppppppppppppppppppppppppppp-namespa-0f932f0421b616
ppppppppppppppppppppppppppppppppppppppppppppppppp (49 chars)	namespace	pppppppppppppppppppppppppppppppppppppppppppppppp-dccac2434fcf48

Related to: #388178 (closed)

database migration

Extracted to: !111613 (merged)

Screenshots or screen recordings

Screen_Recording_2023-01-16_at_19.42.00

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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