Introduce position column on ObjectMetadata

[FelixMalfait]

• add a column position on ObjectMetadata to determine if it should be displayed on the app's left menu and its position
• add a column isDisplayedInMenu (default value false)
• improve @WorkspaceEntity decorator to take into account those two additional positions initialize people/companies/opportunities with big numbers to leave room in the future (e.g. -1000000 0 100000), make sure it's properly initialized for custom objects (added at the end)

Frontend: menu will be re-ordered the same way we re-order favorites (will be done later / in another issue)

[FelixMalfait]

(the primary goal of this is to decorrelate visibility of an object in the app and visibility in settings, it'd be useful to expose some objects in settings without having them appearing automatically on the sidebar)

[FelixMalfait]

Marking as blocked as I feel the separation could also depend on the Layout, depending on our strategy to implement layouts

[FelixMalfait]

Instead we we'll use Favorites to decorelate menu from objects. Not all objects will be displayed in the menu