Allow ordering of models in the admin index

[olivierdalang]

This PR allows to display the models in a specific order per app on the admin index and the app index, using a new order_in_index option on ModelAdmin.

When left to default (None), the models are sorted alphabetically (backwards compatible).

This helps deal with a common case where there is some logical order between models that does not match alphabetical order (e.g. Continents, Countries, Counties, Cities is much more usable than Cities, Continents, Counties, Countries). There is no easy way to achieve this, except by prepending verbose_name with spaces, which is not very elegant.

[felixxm]

Accepted ticket is required for all new features.

[olivierdalang]

Oops my bad, I'm not used to the process yet... I'll open a ticket. Can this stay open in the mean time ?

[hramezani]

Can be related to this ticket

[olivierdalang]

Can be related to this ticket

Thanks hadn't found it. I reopened the ticket, hope this is OK.

[carltongibson]

Closing as per ticket-25671