Refactored and simplified code around flag filters, in preparation for
UI improvements. It's split into 2 PRs in order to simplify what needs
to be behind a flag and what doesn't.
- `ExperimentalColumnsMenu` moved to `ColumnsMenu`, old unused
`ColumnsMenu` removed
- Parts of the code moved to `ProjectFeaturesColumnsMenu`
- Moved `FlagCreationButton` to a separate file
- Removed part behind archived flag (`projectOverviewRefactorFeedback`)
