[IMP] web: allow using many2one as measure in pivot view

With this commit, we add the possibility of using many2ones as measure.
The way a many2one is aggregated is by using the count distinct
operator.

Some care had to be done to gracefully support the situation when a
pivot view is grouped by a m2o field F, and the same field F is used as
a measure.

addons/web/static/src/xml/base.xml

@@ -527,7 +527,7 @@
         </button>
         <ul class="dropdown-menu o_pivot_measures_list">
             <li t-foreach="measures" t-as="measure" t-att-data-field="measure[0]">
-                <a href="#"><t t-esc="measure[1].string"/></a>
+                <a href="#"><t t-esc="measure[1].string"/><t t-if="measure[1].type === 'many2one'"> (count)</t></a>
             </li>
             <li class="divider"></li>
             <li data-field="__count"><a href="#">Count</a></li>