Issue page redesign

Created by: mbajur

Hello! I'd like to help with gitlab project and that's my first bigger commit.

IMO GitLab stuggles a bit in cause of big width of it's layout. On one hand it's a good thing cause a lot of information can be displayed and everything is big and readable but on the other hand, some pages looks really bad. Like issue page for instance. Websites are terrible to read when line has about 50 words in it and current issue page is a great example of that.

I want to dedicate a series of PR focused around this 'big width' thing , trying to fix that my way. I hope you'll like my attempt. If not, no worries.

Just for the start - here is a small redesign of Issue page i've made. It was focused on reducing a text section width by inserting a sidebar. I have also reduced a white-unused-space and moved some things from the top to sidebar. I've also tried to style the notes to make them look more organized. I've tried to make that all as much flexible as possible.

Some things are heavily inspired by github, others not. I hope you'll like them. There is still much work to be done (with issue page and not only) but i'll try to send some more push request regularly.