[GitLab] Allow user-controlled channel targeting

[mweinelt]

For large instances (like Debians) the repository maintainer might want to influence where notifications get sent, without bugging the instance admin. So I propose appending GET variables to the webhook target:

Think:

https://cpthook.example.com/gitlab/?channel=#myproject

[fleaz]

Would this make sense as a general feature for all modules? E.g. the simple module already has this functionality

[mweinelt]

I guess so, even prometheus alerts might only be interesting for smaller subgroups.