[FIX] email_template: when usign the composer in mass mailing mode with
a template, display the user's signature if asked by the template. The use will
then be able to modify the generated content and to remove / update it if necessary;
Otherwise the signature is not added in mass mailing mode, because the content
of the email is based on the wizard's body, not the template one.

Some rewriting in the onchange: now using a browse instead of a read.
[REM] Removed print statement
[FIX] email_template: fixed a test not checking signature but
using a template adding signature.
[FIX] Calendar - Override _name_search to convert virtual_id to real_id
[MERGE] [FIX] email_template: effectively support 'user signature'
in the mail composer when using templates.
[FIX] gamification: do not set a start_date by default as the cron starts past challenges (making data challenge to start) and avoid useless write
[FIX] Calendar - only try to split the virtual id if it's a string