Add core extensions: UserCSS, UserJS (#6267)

* Copy CustomCSS and CustomJS

Original: FreshRSS/Extensions@9f21984

* Rename CustomCSS -> UserCSS

* Rename CustomJS -> UserJS

* Change metadata

The name is used for the directory where the configuration
is stored and should not contain spaces.
Since the name was changed, I reset the version number and
changed to semantic versioning.

* Change data directory

Changed the location of the configuration file to
the user data directory, because it is not `static`.
That way, the user's configurations are gathered
in the user directory, which makes it easier to backup them.

* Edit documentations

Remove procedures to install the extension
because it is no longer necessary.

* Fix wrong variables in the configuration page

Remove permission error indication because the storage location
is now in the user data directory managed by the application.

* Remove the `xExtension-` prefix for core extensions

* Set version to 1.0.0 for UserCSS, UserJS

* Refactoring

* Remove unused variables

* Remove version 0.0.1 in Changelog

Version 0.0.1 will not be merged, so only version 1.0.0 will remain.

* public getFileUrl

* Revert more protected

* Use entrypoint for extension user path instead of name

* Add space to extension name

* Add `#[\Override]`

* Add explains of User CSS and User JS to docs

* Remove README of User CSS and User JS

* Add migration code for extension user path

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Co-authored-by: Alexandre Alapetite <alexandre@alapetite.fr>

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