Fix missing speakers in XLSX export

[Elblinator]

Current behaviour:
If a motion is exported as XLSX then the speaker are always missing from the export

Recreation:

1. create a motion
2. put someone on the LoS
3. export the motion as XLSX and activate "Speakers" in the Meta information
4. in the XLSX-File: the title "open requests to speak" is correctly exported but no speakers are visible

Wanted behaviour:
The speakers in the LoS should be correctly exported, meaning that the amount of participants who wait to speak should be exported

Furthermore: the string "open requests to speak" in the xslx-file should be changed to "amount of open requests to speak"