Unify button styling

[pstorozenko]

ui.input_task_button allows to specify bootstrap type

but input_action_button and download_button don't.
In their case I have to add a second btn- class ui.download_button("btn_download_csv", "Download", class_="btn-primary" ) (now the button has both btn-primary and btn-default) and keep my fingers crossed that their properties don't clash.

I think it would be nice to have all buttons unified.
Can be connected to #1230

What do you think?