Purpose: better showcase the integration between email marketing and crm
application thanks to UTM links.
Specifications
* add 2 stat buttons to the mass mailing form;
* XX Leads -> redirects to lead list view (should include both types: leads
& opportunities), only visible if Leads are activated in CRM settings;
* YY Opportunities -> redirects to opportunities list view (only
opportunities);
* those stats buttons are linked to UTM trackers used in the mass mailing
itself: campaign, source and medium. Those can be configured notably using
the website_link module;
* display the stat buttons even if no data;
* update demo data;
This commit is linked to task ID 1872198 and PR #26188.
