Make route in elks configurable in all places

[iameru]

I make this issue to remind us of refactoring the code later to make this work:

As we mount the Elks router via function and all calls concerning the config need to be in there but the start_elks_call function needs to be outside of this mount function we currently have the elks endpoint string (think {base_url}/{elks} hardcoded.

see this discussion for context

Hint for later: We could clean up the elks routers alltogether and refactor all of this in a class. We might be able to load the functions differently, avoiding the 4 kilometer deep indenting at some parts of the code.