Calculate checksum of the ballot file and write it into meeting minutes

[ApsiV11]

We should be able to verify the ballots also after the election at anytime. Thus we need to have sum sort of a chcksum which we could use to check the ballots.

The requirements for this would be:

1. Calculate a checksum of the ballots file and store it into the meeting minutes
2. Ballots need to be in the same order but not in the order of time of voting
3. The ballot file needs to stay the same always so that the checksum stays the same 2.1. This probably means that we need to store the file into the database since otherwise we couldn't change the file generation in anyway

Thus the process would be end voting -> generate file -> store it in the database (base64 etc.) -> calculate checksum in the UI -> store the checksum into the meeting minutes

[ApsiV11]

If an easy and robust solution is found, this could be implemented but otherwise this is pretty out-of-scope ot this project and our needs.

The db schema already validates the data that malicious data cannot be inserted and the access to the data is restricted to only a few people. One would argue that this system is probably more secure in terms of data changing than current paper voting.