When discuss is not installed, hovering a facet in the searchbar makes
the cog overlap the icon/text.
This is due to the `o-bg-inherit` class which is set in the mail module.
Therefore if discuss is not installed, the SCSS for the background
doesn't exist.
This commit moves the `o-bg-inherit` as a new utility class `bg-inherit`
, allowing it to be used even if no apps are installed.
Therefore also renaming the occurences of `o-bg-inherit` into
`bg-inherit`.
task-3575837
closesodoo/odoo#140334
Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>