Added the option to show a delete confirmation modal

[abeaugustijn]

Hi there,

Thanks a lot for continuing this great project, it's nice to have the ability to self-host well built services.
Somehow some of my users have problems with accidentally deleting uploads, so I added the ability to show a confirmation modal, which looks like this:

It is disabled by default but can be enabled by setting SHOW_DELETE_CONFIRM to true.

I added translations for English, Dutch and German. How should we handle getting the rest?

Let me know if there are any changes I still need to make.

Thanks in advance,
Abe