Storage migration of elections-phragmen (#3948)

* Initial sotrage migration

* Fix some deps

* test added

* another dep removed

* Update srml/elections-phragmen/src/lib.rs

Co-Authored-By: Bastian Köcher <bkchr@users.noreply.github.com>

* Apply suggestions from code review

Co-Authored-By: Bastian Köcher <bkchr@users.noreply.github.com>

* a bit nicer