enforced key backup setup

[yasinishyn]

This PR is an example of how https://github.com/vector-im/element-web/issues/23810 issue could be fixed. I am not adding tests for the purpose of seeing if this might gain tracking in the community

In terms of functionality it has two-fold:

1. Redirect to encryption setup steps if the backup isn't set;
2. hired "skip" buttons inside the backup flow windows.

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This PR currently has none of the required changelog labels.

A reviewer can add one of: T-Deprecation, T-Enhancement, T-Defect, T-Task to indicate what type of change this is, or add Type: [enhancement/defect/task] to the description and I'll add them for you.

[dbkr]

Making this a draft while it's still in progress.