Put all sidebar icons in fixed width container

## What does this MR do?
Puts all icons (font awesome and custom) in fixed width container to fix weird alignment

## Why was this MR needed?
Mix of font awesome icons & custom icons results in different icon widths

## What are the relevant issue numbers?
Closes #18539 

## Screenshots (if relevant)
<img src="/uploads/94a82258d5d71df0b9608ccd4d7e5cd1/Screen_Shot_2016-06-14_at_9.57.25_AM.png" width="400px">



See merge request !4648