Add "Manage segment overrides" environment permission

[luk355]

Segment overrides allows release of a specific feature to broader set of users. Not all Flagsmith users should have access to update this settings, therefore, I would be great to have dedicated "Manage segment overrides" environment role which would limit this.

This feature would also work well with the 4 eyes as segment overrides are not hidden behind the feature flag.

Acceptance Criteria

• Users and groups can be given a new 'Manage Segment Overrides' permission
• Non-Admin users with the 'Manage Segment Overrides' permission in a given environment are able to add, edit and remove segment overrides for flags in that environment
• Non-Admin users without the 'Manage Segment Overrides' in a given environment are able to view segment overrides for flags in that environment
• Non-Admin users without the 'Manage Segment Overrides' in a given environment are not able to add, edit or remove segment overrides for flags in that environment

[dabeeeenster]

This sounds sensible to me - thoughts @gagantrivedi @kyle-ssg @matthewelwell ?

[matthewelwell]

Yes, for sure.

[zachaysan]

The backend work for this feature is nearly complete. During testing I determined that we are going to need some frontend rule changes to make the interface work as expected for our customers.

Right now the segment overrides section of the interface is still active looking if a user has the ability to edit features. Since this is a different permission, however, we'll now need to check if the user has the MANAGE_SEGMENT_OVERRIDES permission and only show a read-only version of the interface if they do not have it.

In addition to the above, please also hide the Create Feature-Specific Segment button if the user is missing the project permission MANAGE_SEGMENTS since they will not be able to create a segment if that permission is missing.