Pages Domain Limits

What does this MR do and why?

Fixes: https://gitlab.com/gitlab-org/gitlab/-/issues/244293

Add ability to limit to the number of custom domains that can be associated with each GitLab Pages site. The default has been kept as 0 which corresponds to unlimited to make this change non-disruptive.

Screenshots or screen recordings

Configuring the limit

No longer able to add "New Domain" when limit is reached

**When you try to browse the "create-new-domain" URL directly

URL - <PROJECT_URL>/pages/domains/new

How to set up and validate locally

Follow the steps in the documentation(index.md) changes in this MR

Migrations

Up

main: == 20220818095225 AddMaxPagesCustomDomainsPerProject: migrating ===============
main: -- column_exists?(:application_settings, :max_pages_custom_domains_per_project)
main:    -> 0.4001s
main: -- add_column(:application_settings, :max_pages_custom_domains_per_project, :integer, {:null=>false, :default=>0})
main:    -> 0.0034s
main: -- transaction_open?()
main:    -> 0.0000s
main: -- current_schema()
main:    -> 0.0007s
main: -- transaction_open?()
main:    -> 0.0000s
main: -- execute("ALTER TABLE application_settings\nADD CONSTRAINT app_settings_max_pages_custom_domains_per_project_check\nCHECK ( max_pages_custom_domains_per_project >= 0 )\nNOT VALID;\n")
main:    -> 0.0022s
main: -- current_schema()
main:    -> 0.0006s
main: -- execute("ALTER TABLE application_settings VALIDATE CONSTRAINT app_settings_max_pages_custom_domains_per_project_check;")
main:    -> 0.0019s
main: == 20220818095225 AddMaxPagesCustomDomainsPerProject: migrated (0.4185s) ======

Down

main: == 20220818095225 AddMaxPagesCustomDomainsPerProject: reverting ===============
main: -- column_exists?(:application_settings, :max_pages_custom_domains_per_project)
main:    -> 0.4074s
main: -- remove_column(:application_settings, :max_pages_custom_domains_per_project)
main:    -> 0.0027s
main: == 20220818095225 AddMaxPagesCustomDomainsPerProject: reverted (0.4104s) ======

MR acceptance checklist

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