Committee export

[Elblinator]

Current behaviour:
If a committee is exported then some field are missing, in the wrong format or otherwise not correct

Wrong format in the CSV export file:
if committees are exported then:

• the managers are exported with their first and last name, but the import needs their username
• the meeting_start/end_time is exported as YYYYMMDD, but the import needs the date to be YYYY-MM-DD
• the meeting admins are not exported
• column meeting_template is missing and should be added (New requirement added by MSoeb 20.03.2024)

[jsangmeister]

Also, the columns are named wrong. They should be named according to the field names defined in the backend

[reiterl]

Need to fix the third point. I have no idea, how to do that, so I rm my assignment here.