Improve link preview code
* remove useless code indirections;
* improve performance of link creation, cleanly support batch-creation (could
lead to 98+5 URLs to the same domain creation within default 10 seconds
instead of 98+1 but we feel it's ok);
* name methods according to ORM;
* use tools for html empty;
Also cleanup link preview tests: concatenate tests when possible, improve
setup, use real-life scenarios (using 'message_post' notably), remove useless
data creation.
Task-3557985 (Mail: Message cleanup (shortcode, link))
Prepares Task-36879 (Mail: Support MultiCompany Aliases)
Part-of: odoo/odoo#138938