Expose modRewriteWorking via capabilities to allow pretty URL by clients

[ShGKme]

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Is your feature request related to a problem? Please describe.

Different clients (Files, Talk) sometimes need to generate links. Without knowing about modRewriteWorking, they need to always generate links with index.php.

This does not allow to pretty URL even when it is supported by the instance.

Describe the solution you'd like

Provide modRewriteWorking via core capabilities, not via OC.config.modRewriteWorking JS variable.

Describe alternatives you've considered

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Additional context

• Pretty URL for right-click copy link desktop#3766
• Pretty URL for conversations link talk-desktop#605