Add new icons in left sidebar and layout nav

Closes #17609    
Closes #17240    
Closes #17321    

![Screen_Shot_2016-06-02_at_8.47.38_AM](/uploads/f254fe81cba65cbdeb8e5720f0b2fc19/Screen_Shot_2016-06-02_at_8.47.38_AM.png)
![Screen_Shot_2016-06-02_at_8.53.33_AM](/uploads/2c27284f0a4916fe447794d9b663c4df/Screen_Shot_2016-06-02_at_8.53.33_AM.png)

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

See merge request !4268