[FIX] base: BS4, restore <select/> style
[FIX] base: BS4, restore <select/> style

In enterprise version, the select elements have a custom style. For it
to work, the "appearance" CSS property must be understood... which is
not the case in most browser without vendor prefixes.

As discussed in https://github.com/odoo/odoo/commit/770aec399c4e6e29cfc5854318cbda19a605315d,
vendor prefixes are added by post-processing the scss generation.

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