Add jinja template helper

[Josue-T]

Problem

In theses following case it's quite complicated to implement with the current templates

• Multi line entries
• conditional block

And the yunohost implementation is not a standard template system

Solution

Use jinja as already used in the core to bring many feature.

I've implemented this on monitorix cf https://github.com/YunoHost-Apps/monitorix_ynh/pull/55 and synapse cf https://github.com/YunoHost-Apps/synapse_ynh/pull/449 and this bring many code simplifications and it just more clear how it work than without this.

[alexAubin]

(Closing because now available in the core with --jinja on ynh_add_config)