Add new icons in left sidebar and layout nav

Annabel Dunstone Grayrequested to merge
17240-svg into master

Closes #17609 (closed)
Closes #17240 (closed)
Closes #17321 (closed)

Screen_Shot_2016-06-02_at_8.47.38_AM Screen_Shot_2016-06-02_at_8.53.33_AM

@skyruler I couldn't find the icons for Projects, Merge Requests, Snippets, or Help in the design repo. Do you have them available somewhere?

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