[IMP] crm: prohibit calendar lead creation

Currently the 2 CRM calendars (Pipeline and Leads) only display leads
when they have a linked activity. Therefore creating new leads within a
calendar view was confusing since users could not see the new leads
without switching to another view (note that new leads could not have
activities linked to them under current design and expecting users to
know to add an activity to a new lead to immediately view it in the
calendar view is unreasonable.)

This commit makes it so users can no longer add new leads to calendar
views in crm module. Current behavior of being able to edit leads
already in calendar is untouched.

Part of
Task: 2126530

addons/crm/views/crm_lead_views.xml

@@ -392,7 +392,7 @@
             <field name="model">crm.lead</field>
             <field name="priority" eval="2"/>
             <field name="arch" type="xml">
-                <calendar string="Leads Generation" mode="month" date_start="activity_date_deadline" color="user_id" hide_time="true" event_limit="5">
+                <calendar string="Leads Generation" create="0" mode="month" date_start="activity_date_deadline" color="user_id" hide_time="true" event_limit="5">
                     <field name="planned_revenue"/>
                     <field name="partner_id" avatar_field="image_128"/>
                 </calendar>