Add Hover animations

For every element that has a hover treatment, we should add a nice transition animation. While this is subtle, it will help the entire experience feel more polished and professional.

This animation should be a 200ms linear transition. See details and examples here: https://docs.gitlab.com/ce/development/ux_guide/animation.html#hover

I did a quick search for hover in our CSS folder to try and compile a list, but I probably missed things. I suspect we won't get everything in the first pass, but I'm sure we can get a good start 😄

• Awards buttons
• Buttons
• Cell on User contributer Calendar (this works with svg and it therefore harder to animate.. will leave this for now)
• Dropdowns
• Rows in File list (under Repository)
• Links
• Navigation tabs
• Icon commands (such as bold, italic, etc. in a markdown comment field, edit, delete for a comment in an issue, copy to clipboard buttons)
• Cycle analytics stage tabs
• Text fields & Search fields
• Pipeline status badges
• labels (also add hover state)