Improve the log_params_changes() for boutiques tasks

[prioux]

When CBRAIN a task is updated by a user and the user changes some of the params, the method log_params_changes() is called to make a 'pretty' log of what has changed. This happens in the update action in the tasks controller. The method was created a long time ago, well before we had tasks implemented through boutiques descriptors. The new boutiques tasks store all their params inside a single key in the params, the "invoke" structure. So the log_params_changes() is too dumb and when a single boutiques parameter changes it will report that the entire invoke structure has changed, with before and after.

We need to make the method 'aware' of the invoke structure, if present, and make it more intelligently report what has changed in it.