Fix padding for icons in DropLab dropdowns

Summary

As per https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16854#note_57362016, dropdowns should generally have 8px between icons and text.

Steps to reproduce

• open search bar on issue list

What is the current bug behavior?

Not enough space between icons and text:

What is the expected correct behavior?

8px space

Proposal

All the icons should have a 16x16px bounding box and the margin between the icon and the text should be 8px.

Search dropdown	Bounding box	Spacing