REST API: Unify SiteID specifications

[martastain]

This PR unifies site_id dependency across rest endpoints. Two dependencies SiteID and ClientSiteID are used as specified in the linked issue.

Discussion & future

Endpoints /api/projects/{project_name}/roots/{root_name} still use site_id provided in path. This is inconsistent, but probably not worthed breaking the backwards compatibility.

/api/settings still expect site id to be provided as a query argument. Should it also accept header option as it is called from the client application and site_id is by default sent with the request? or do we need granularity here and option to get just project settings? @iLLiCiTiT

Testing

• Save project site settings and ensure they are correctly stored when different sites are used
• Save different site overrides for project roots
• Ensure site overrides for settings and roots are correctly propagated to the client

[antirotor]

Thanks, it works for my use cases