Homogenize the variables that can be used in the email templates sent by the forms.

Currently, the following variables do not work in all models: --FIRSTNAME--, --FIRSTNAME--, --FIRSTNAME--, --GROUPMANAGER--

Is it possible to add the variables:

--DEBUTVALIDITYFORM--: the start date of validity of the form (= start of registrations)

--END OF FORM VALIDITY--: the end date of validity of the form (= end of registrations)

In the case of a form attached to a one-time or regular event, it would be nice to add the following variables to all models:

--ACTIVITYNAME--: the name of the one-time or regular activity

--DEBUTACTIVITY--: the start date of the one-time activity/the date of the next session of the regular activity

--FINACTIVITY--: the end date of the one-off activity/the date of the last session of the regular activity

--ACTIVITY MANAGER--: the name of the person in charge of the one-off or regular activity