Make Static_URL first path configurable, they are now hard coded in init.go

[fredvd]

The links where Listmonk received Tracking information, like View Tracking and Link tracking, are currently based on the RootUrl, but their paths are hardcoded.

https://github.com/knadh/listmonk/blob/296245a1b3fecea39d47784ec903cb26ed207084/cmd/init.go#L416-L433

In some of our setups, we operate Listmonk on an internal domain, and only expose the exact links on the organisation / website domain. It would be great to avoid path collisions, if the /campaign/, /link/ and /subscription/ paths in the Static Urls could be configured using a similar setting as the RootUrl.

[knadh]

Hi @fredvd. That would be a very niche (and arbitrary) feature to have a configurable URL for a handful of selected paths.

We have a similar setup in prod and we've rewritten those URLs at the reverse proxy level in Nginx. So site.com/link/... from the outside is rewritten and proxied to listmonk.internal/link/...