Add placeholder images on talkcards + event page

[RemyMess]

3 ways of adding logic for placeholder images

1. Populating images/placeholders/[mathematics, biology, artificialintelligence, ...] folder and create a duplicate in covers/avatars when channel is getting created
2. Populating images/placeholders/[mathematics, biology, artificialintelligence, ...] folder + change logic of "has cover" into integer (0 = no, 1 = yes, 2+= id of the image in the placeholder folder)
3. Add lists of URLs (https://images.pexels.com/photos/1292998/pexels-photo-1292998.jpeg?auto=compress&cs=tinysrgb&dpr=3&h=100&w=100) + add URL image in DB

Final solution:

Use pexels API.

Todos:

• [ ] Add placeholder images in talkcards
• [ ] Add placeholder page in eventpage
• [ ] Add placeholder images in channelpage
• [ ] Check if it's slow to load