Replace open/closed issue icons with new ones

Item Before After
Open issue issue-open-m.svg Frame_45.svg
Closed issue issue-close.svg Frame_93.svg
Original description Screen_Shot_2019-02-13_at_16.52.39

Per @markpundsack .

Per Victor: I can't totally recall my initial reactions to these icons. But I currently I don't find them confusing, but I use GitLab waaaaay too much so I'm biased.

Edited by Annabel Dunstone Gray