Add selection rule option

Problem/Motivation

It is useful to have ability to set domains per environment and switch between them.
For example you have next language domains settings:
English: example.com, dev.example.com, staging.example.com
German: example.de, dev.example.de, staging.example.de

Then if you choose "Domain with equal order index" setting in "Domain selection rule" your English dev domain will switch to German dev domain, stage to stage and etc. All you need is to put domains per environment in the same order.

Proposed resolution

Patch with proposed resolution attached in comments.

User interface changes

New checkbox on language edit page.

Data model changes

New variable language_multidomain_selection_rule to keep selection rule.