fix(LegalVote): update participant selection to use id over index

In order to reproduce the issue, you need 2 legal vote eligible users in the meeting. Search for the 2nd user in the list in such way that the first one disappears. Since we relied on the index before, 2nd one became 1st and clicking on him doesn't change the checkbox, but when search is cleared we observe that the first one is checked.

With this merge request, we switched from using index to select participant to their ids.

Closes #1832