Start train_model with multiple annotations

[normanrz]

Detailed Description

Add a new modal in the "AI models" list to start a new training. The modal should have a selector (in the simplest case a number of text boxes for URLs or IDs) to select annotations to train with. The other options of the existing train model modal should also be included, if possible.

Context

Training new models from annotations is already a breeze. However, we often times want to train with data from multiple annotations (e.g. to include data from multiple datasets).