roadmap: change structure

The structure of the roadmap isn't meant to support more than 6 tabs, so some changes are necessary to show it correctly. I added some changes since i was at it.

• Grouped the first 2 years 2014-215 in a single tab.
• Added headers for each card
• The last tab is now "Future", and doesn't refer to a specific year, but it's meant to show updates that will be implemented in the future
• Changed default card from 2019 to 2020 This PR is needed to show the current roadmap.

Preview: