JS advertisement type does not yet actually run the JS

We should also add an explicit permission to enter / edit the JS for security reasons.

As discussed with @anybody, I have moved all code related to the JS ad type into a separate submodule, and we will tackle the proposed solution there.

Merging current state so that we can then finish this in a follow-up branch.

@grevil: Since moving the configs over into the sub-submodule, the tests are broken - they worked before. Any idea? Maybe a config dependency we overlooked?

Everything is done now, apart from finding the reason of the test failure. But as far as I understand, @grevil will take a look at that? So I'll set this to review now.

I'll quickly add a conditional message if the user is lacking the permission to edit the JS field.

It currently appears to be near impossible to change the field description in the alter hook, as every attempt I have pursued at changing it seemed to have no effect. I have committed my best attempt at this time, which successfully updates the description in code, but does appear to cause any change in the displayed text.

@grevil, perhaps you know a proper solution to this?