* Pages are enabled by default on each project
* Add note about using the `only` parameter

- Move UploadsTransfer to ProjectTransfer and inherit from this to UploadsTransfer and PagesTransfer

Revert "Store pages in shared/pages/fqdn/fqdn/public or shared/pages/fqdn/subpath/public - makes it simpler to implement CNAMEs in future"

This reverts commit 86a2a78f0d13a678899460638add6b862059433e.

This reverts commit 0d73bd93bab349d84910cf14773633b8a66df466.

Store pages in shared/pages/fqdn/fqdn/public or shared/pages/fqdn/subpath/public - makes it simpler to implement CNAMEs in future

- Fix markdown
- Remove how it works section, maybe add it at a later point

- The pages are created when build artifacts for `pages` job are uploaded
- Pages serve the content under: http://group.pages.domain.com/project
- Pages can be used to serve the group page, special project named as host: group.pages.domain.com
- User can provide own 403 and 404 error pages by creating 403.html and 404.html in group page project
- Pages can be explicitly removed from the project by clicking Remove Pages in Project Settings
- The size of pages is limited by Application Setting: max pages size, which limits the maximum size of unpacked archive (default: 100MB)
- The public/ is extracted from artifacts and content is served as static pages
- Pages asynchronous worker use `dd` to limit the unpacked tar size
- Pages needs to be explicitly enabled and domain needs to be specified in gitlab.yml
- Pages are part of backups
- Pages notify the deployment status using Commit Status API
- Pages use a new sidekiq queue: pages
- Pages use a separate nginx config which needs to be explicitly added

Fix user validation error in Import/Export

Closes #23833

See merge request !8909

Don't capitalize environment name in show page

Closes #27484

See merge request !8902