Allow multiple url_template in version switcher

[tupui]

url_template allows to have a template to build the URL for each versions. The problem is that old versions or development ones can be stored elsewhere and not respect the template.

I can see a few options:

• Add an optional url in the json configuration which would overwrite url_template.
• Allow multiple templates in conf.py and add a way to select which template has to be used. Could be a regex with the version/name, another optional parameter in the json, etc.
• Have another file which would define mapping exceptions.

A use case with discussions can be seen in scipy/scipy#15380

[jorisvandenbossche]

I suppose having an additional, optional entry in the json file might be the cleanest solution.

Another issue about potentially expanding the json file: #556

[drammock]

Add an optional url in the json configuration which would overwrite url_template.

I like this option best too.

[tupui]

If it's just a matter of adapting the current JS in the HTML (and doc OC), I could give this a try 😃
I've seen the issue about moving the JS out of the HTML, since my JS/HTML skills are "practical" I don't know if adding new features can be done before or if it's better to wait for the move.

[jorisvandenbossche]

Yes, I think it's certainly fine to improve the current JS, before it is moved. So if you want to give this a try, that would be very welcome!

[tupui]

@jorisvandenbossche I created a PR (and another one for the extra classes).