FIX / Add project template selection modal and clarify template behavior

[Mary-Clb]

• I have read the CONTRIBUTING document.
• I have performed a self-review of my code.
• I have added tests that prove my fix is effective or that my feature works.
• This change requires a documentation update.

Description

It fixes !40258
The project creation via templates workflow had a confusing UX where users could add elements as tasks and teams to templates, thinking they were creating actual projects.

The issue was resolved by implementing 2 elements :

• Bootstrap modal dialog that appears when users click on any project template for creation : the users must explicitly confirm their intention before project creation proceeds.
• Warning banners in the project form when users are in template context, explaining they have to first register the project before adding any elements, as otherwise they will only be created on the template.

##Screenshots

[trasher]

All strings should be translatable, and redacted in english.

[trasher]

Not sure on the feature itself;, but that's the same for all "templates"; not only for projects.
While I'd agree the UI should distinct a bit; displaying huge warnings is probably counter productive.

[Mary-Clb]

Replaced by #22293