Fixed issue with bold in issuable sidebar

## What does this MR do?

~~The sidebar contained both normal font weight & bold font weight, so this standardises them to bold.~~

After looking at the designs, i've updated the font weights & colors throughout the sidebar to correctly match them

## Screenshots (if relevant)

![Screen_Shot_2016-06-02_at_11.03.40](/uploads/0a3eb6a67ce5722c77f6adf2fe883017/Screen_Shot_2016-06-02_at_11.03.40.png)


See merge request !4398