[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.

odoo/addons/base/models/assetsbundle.py

@@ -422,6 +422,8 @@ class AssetsBundle(object):
         css_content = '\n'.join(asset.minify() for asset in self.stylesheets)
 
         # Post process the produced css to add required vendor prefixes here
+        css_content = re.sub(r'(appearance: (\w+);)', r'-webkit-appearance: \2; -moz-appearance: \2; \1', css_content);
+
         css_content = re.sub(r'(display: ((?:inline-)?)flex((?: ?!important)?);)', r'display: -webkit-\2flex\3; \1', css_content)  # For PhantomJS tests
         css_content = re.sub(r'(flex-flow: (\w+ \w+);)', r'-webkit-flex-flow: \2; \1', css_content) # For PhantomJS tests
         css_content = re.sub(r'(flex: (\d+ \d+ (?:\d+|auto));)', r'-webkit-flex: \2; \1', css_content)  # For PhantomJS tests