Avoid URL field for themes

[Guite]

The URL field should be hidden (and not required) for themes.

cc @gfr

[craigh]

why? you are even using one in the bootstrap theme.

[Guite]

Right. But it shouldn't be required then, should it?

[Guite]

If it should be required, then we need to add it to the theme's composer.json, like we do it for modules, too.

[craigh]

so, I don't think you actually used one in BootstrapTheme. (it appears the route is simply /ajax/foo instead of /bootstrap/ajax/foo

I don't see this as a bug. It is functional as is.

What case brought this up?

[Guite]

No specific case. It is just confusing if you want to edit the theme data and don't know what you need to input in the field which is empty but required.

[craigh]

ah - I see. then it is not functional as is.

still, the problem is bigger than a UI concern.

[Guite]

This will be done in two steps:

1. make the URL input field optional in 3.0.1 (but only for themes) in order to make the input form less confusing for people editing a theme.
2. postpone the issue for making the URL a requirement for themes meta data in 4.0.0 (create a different issue for that).