Improved the Accessibility of the tool

[killerrin]

Hey there,

Our company recently had an accessibility audit done on tools that are currently in use, and LevelUp came back with a few faults. The issues were pretty quick to resolve, so I figured I would write some fixes and submit them in hopes that we could get them added into the extension.

Within this pull request contains all of the enhancements identified:

• Adding aria-hidden to all the Icons so that a Screen reader doesn't attempt to read them out
• Adding aria-describedby to all of the buttons so that their tooltips are properly read out by screen readers
• Added underlines to the links in the top-right corner to make them visually identifiable as links
• Fixed a colour contrast issue inside of the "Impersonate" tab to make it WCAG complaint
• Added a background colour to the Buttons so that a user who is using their keyboard can visually identify at a glance when a button is selected (the current buttons have a drop-shadow, however users with a visual impairment would find it hard to see)
• Added an aria-label to the Drawer Button in the top-left corner so that a Screen Reader would read out "Admin Area" instead of just "button"

It'd be awesome if we could get these changes in.