Use a modal in worker configuration model fields

Refs https://redmine.teklia.com/issues/6877

In worker configurations, model fields are currently using a dropdown to show the available model values. Those dropdowns do not mix well with modals, and we had to make compromises with them that prevent using them in this particular case (!1246 (merged)). When a worker configuration has enough fields to require scrolling, the dropdown does not scroll along with its input, so the suggested models are not visible.

To replace the dropdown, we will instead use a modal to pick a model. This is what was being done in Training processes before we removed them. We can restore the ModelPicker component and turn it into a user configuration field. Clicking on the input opens the modal, allowing to select a model, which is then displayed in the field. The generated JSON should not change; we will still have the model UUID as the field's value.