releases: Add support for gitlab

[Fumesover]

Allow the release module to target both github and gitlab by making it target a generic "forge" (to allow adding other git forges later)

The default source is set as github and it won't cause any issues for existing install when upgraded.

[svilenmarkov]

Hey, thanks for contributing!

I think rather than having to choose a single source and adding multiple widgets for each source, what might make more sense is to be able to choose the source for each repository individually. So, rather than this:

- type: releases
  repositories:
    - immich-app/immich

- type: releases
  source: gitlab
  repositories:
    - inkscape/inkscape

Something like this which defaults to GitHub but allows you to specify a prefix for other sources:

- type: releases
  repositories:
    - immich-app/immich
    - gitlab:inkscape/inkscape