Enable group wikis import/export [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

In !51873 (merged) we merged all the necessary logic to import and export group wikis.

In this MR we enable group wikis' import/export functionality by removing the feature flag.

We also add the UI changes to reflect it.

The doc MR is in !53250 (merged)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content

Related to #300303 (closed) and #207886 (closed)