Sort for candidates

[Elblinator]

resolves #5391
resolves #6160

Additionally I added the number behind the assignment-list this was a wish from @emanuelschuetze

@xela1601, I used your implementation but changed some functionality, added some features
and added three different variants of the visuals.
We will decide which visuals to use and then I'll remove accordingly

[MSoeb]

Sorting works fine and fast. Two points needs to be changed.

1. Small ranking improvement needed: If the candidate list contains of accounts that only have a first name or a last name, the sorting doesn't work properly.

Example - sorting by first name:

First account 'executive board' is only a last name and it will be ranked on top position. That is wrong. The account 'executive board' should be at last position. - This problem also occurs if an account has only a first name and the sorting is done by last name. In this case, the account with only a first name should also appear on the last position.

2. Add sort button instead of separted buttons. Options should be first name and last name. If sorting is active highlight the word sort. No active sorting, the word sort should not be highlighted.

[MSoeb]

Looks good. Nice :)