Change form types on training interface

[ledell]

• Change target column from drop-down to picker; It's much nicer since some datasets will have a lot of options.
• Change max_models and max_runtime_secs to a text field.
• Or consider a spinbox with a huge upper limit for the values: https://wave.h2o.ai/docs/examples/spinbox, and if that is not good, let's just do something like this: Add form validation for inputs that should be numeric: https://wave.h2o.ai/docs/widgets/form/textbox/

[ledell]

Fixed by: #40