Custom Javascript

**Is your feature request related to a problem? Please describe.**
I'd like to be able to set different CSS per tenant and sometimes even per flow. I would also like to further customise the functionality of certain things for my specific application.

**Describe the solution you'd like**
I think it would be pretty straightforward to inject a custom javascript file just like the optional custom css file that gets injected on every page. This would take almost no work to implement and would open up basically infinite potential for customisation.

**Describe alternatives you've considered**
I thought about just adding this to my own install but decided against it as it would complicate updates.