[FIX] web: limit on search panel values

Before this commit, if a many2X field with a big comodel was added
in a search panel, the view using it would crash. For instance that
problem occured in the kanban view for hr.job, where res.users
appears as the comodel for the field user_id.

Now, we fix an arbitrary limit of 200 to the numbers of values to fetch
for each many2X fields in the search panel. This avoid the problem
mentionned above.
Furthermore, in case the limit is attained for a field used
as select="one", the values are displayed without being hierarchized.
Indeed the limit can leads to gaps in the knowledge of the hierarchy
and consequently to a bad representation of it.

Task ID: 2154668

closes odoo/odoo#49857

Signed-off-by: Aaron Bohy (aab) <aab@odoo.com>