Update issue list icons

Problem

The icons at the top of the issue list have the following problems:

• Mixing FontAwesome (RSS, calendar) icons with our custom SVG icons
• Export is using the download icon
• Import is using the upload icon
• The calendar icon is using #000000 and should be #2e2e2e

Solution

• Use and from the SVG library
• Use from the SVG library for import
• Use from the SVG library for export
• Use #2e2e2e for the calendar icon color