Fix initialization order breaking registerFilter in customjs

[KCMertens]

If you want to use the insertBefore property, the other filters need to be registered. But that happens only after the corpus data has loaded.
In the previous frontend there was a callback you could register, but they were removed.

• As a stop-gap reintroduce these so old scripts don't suddenly break.
• For the next major/vue 3 we should probably migrate to the system Jan has set up where everything is a callback.