[REF] *: BS4, remove the use of btn-sm

Odoo made the bad choice of using the 'btn-sm' class for every button
instead of configuring the padding for default 'btn' to be smaller.
In BS4, the style of btn-sm is actually more complex, lowering the
font-size too. Also, btn-xs was removed so we would not have the
possibility to display smaller button than our default ones.

This commit removes btn-sm wherever it was used. Unfortunately, this
might remove it at some places where it made sense but this can be
restored in a second time.