Invitation E-Mail: Add more placeholders

[MSoeb]

Current behavior
The invitation mail misses some placeholders to cover various formulation requirements.

Wanted behavior
Please add placeholders for:

• Title (of account)
• Given name
• Surname
• Structure level (if possible to add as placeholder)
• Groups (analogous to structure_level)

--> Admins should have the possibility to add these placeholders to the invitation mail.

Additionally the gray help text in settings > participants > Email > Email Subject should be rewritten to include the new placeholders.
New text: You can use {event_name}, {username}, {title}, {given_name}, {surname} and {structure_level} as placeholder.

[peb-adr]

In OpenSlides/OpenSlides#6022 it was also called for groups to be added to the list of placeholders.
I'll edit the original comment of this issue to include this.
We discussed that if groups can be included it should be doable analogous to structure_level.

[peb-adr]

I also want to remark: The placeholder strings should be analogous to the field names used in the models.yml. This way they are more predictable (especially to the more technical knowledgeable users)

Additionally the gray help text in settings > participants > Email > Email Subject should be rewritten to include the new placeholders. New text: You can use {event_name}, {username}, {title}, {given_name}, {surname} and {structure_level} as placeholder.

This should become: "You can use {event_name}, {username}, {title}, {first_name}, {last_name} and {structure_level} as placeholder."