Revert "Another commit with many small fixes [Urgent]"

Reverts fossasia/fossasia.github.io#140

There are too many small changes. I find it difficult to preview everything. Maybe you could set up a test website on github for so many changes. I understand that there are different views on design, but we also need to take care that different students are contributing. I see three issues here:
- the twitter area is deleted and substituted with the standard embedding
- background colors of mentor and student section gone 
- background color of bottom section is gone