Requests for comment/Custom inter-namespace tabs

This is a request for comment about adding custom inter-namespace tabs (13228).

Background
Certain wikis currently have an additional namespace tied to the classic (and traditional) article–talk combination. For example, on the English Wikinews, there's a third tab for "Opinions" which leads to a corresponding page in the Comments namespace.

The current implementation has issues, namely:


 * it's in JavaScript (n:MediaWiki:Comments.js) which means that
 * it's available only to JavaScript-enabled browsers;
 * it's doing an extra AJAX request for tab color; and
 * it's a pain in the ass to localize and customize per project.

Use-cases

 * "Opinions" tab for the Comments namespace on the English Wikinews
 * "Citations" tab for the Citations: namespace alongside the ns:0 and Talk namespaces on English Wiktionary

Considerations

 * Put code in a MediaWiki extension or in core???
 * Localizable tab name
 * Configurable in LocalSettings.php with an array or something?
 * Tab has to account for page existence

Plan
The considerations above need to be resolved. Once they have been, this should no more than an hour's worth of coding time, I think. It'll be much more time trying to get the code reviewed and deployed than it will be the write the damn feature.

There's code at User:Svippong/AdditionalTabs and at , also #37569. Given changes to MediaWiki, however, it may make sense to start from scratch. Again, it depends how this is going to be implemented (extension or in core).