Filter tasks by offliner

[benoit74]

At least at the API level in the /tasks endpoint, being able to filter tasks by offliner (wikihow, zimit, ifixit, ...) would be help dev users analyzing the production behavior.

@Popolechien @RavanJAltaie do you think you could benefit from such a filter at the UI level in the https://farm.openzim.org/recipes page? Anywhere else?

@kelson42 @rgaudin your feedback is also welcomed 😄