Activities: Section header links & buttons

[dandineen]

Add in clear hover, active and focus states. The Material defaults aren't very easy to see.

In this screengrab the 'Add' button is actually active but you can't really see it!

The basic hover styling is defined in Figma: https://www.figma.com/file/LN4ZEEGC8E1JeZZwDHSOGO/Activities-https-trello.com-c-qiUa5KP1?node-id=87%3A287

Product/UX will specify styling for focus and active states.

Make sure both the icon and the label are active and clickable on all section headers. I've only spotted this issue on the Employment (+) Add button but it might be present on other headers.

[dandineen]

I've updated the Activities Figma file with a full set of styles for the section headers.

https://www.figma.com/file/LN4ZEEGC8E1JeZZwDHSOGO/Activities-https-trello.com-c-qiUa5KP1?node-id=87%3A287