Recompilation speedup: make Router a runtime dep of FrontendStatic

#2651 (closed)

Builds off !3419 (merged) to remove Pleroma.Web.Router as a compile-time dep of FrontendStatic.

This gets rid of 10 compile-time cycles: https://www.diffchecker.com/KAVfbFac

Pleroma.Web.Router was used in a module attribute, causing it to recompile. I've moved it to a local variable instead so it becomes a runtime dep.