Make start date NOT editable when there are voters

[TheoSabattie]

I got a report on support, people was not able to edit election due to an error

After adding some log I saw that was coming from following error: "Cannot change the start date of an election that already has votes."
That was coming from something else I fixed.. However, I consider we should make the start date option not-editable to prevent user trying to apply a change on start date that is not possible (user will get error later from server). (should be locked + we should add information message: "Start date cannot be changed since you got some votes")