Timeline: Fix icons and assets

I fixed them in PRs that were merged after the JSON used for this conversion was generated.

Author: hedyhli

pydis_site/apps/timeline/entries/2020-04-14_40000-members.md

@@ -5,4 +5,4 @@ icon: fa fa-users
 icon_color: pastel-dark-blue
 ---
 
-![](https://cdn.discordapp.com/attachments/354619224620138496/699666518476324954/unknown.png)
+![](https://raw.githubusercontent.com/python-discord/branding/main/wallpapers/bigger%20than%20liechtenstein/pydis%20postcard.png)

pydis_site/apps/timeline/entries/2020-12-01_advent-of-code.md

@@ -1,7 +1,7 @@
 ---
 title: Advent of Code attracts hundreds of participants
 date: December 1st - 25th, 2020
-icon: fa fa-snowflake-o
+icon: fa fa-snowflake
 icon_color: pastel-red
 ---
 

pydis_site/apps/timeline/entries/2021-03-21_summer-code-jam-2020-highlights.md

@@ -1,7 +1,7 @@
 ---
 title: Summer Code Jam 2020 Highlights
 date: Mar 21st, 2021
-icon: fa fa-youtube-play
+icon: fab fa-youtube
 icon_color: pastel-red
 ---
 

pydis_site/apps/timeline/entries/2023-07-11_new-paste-service.md

@@ -1,7 +1,7 @@
 ---
 title: Switch to new paste service
 date: Jul 11th, 2023
-icon: fa-regular fa-clipboard
+icon: fa-solid fa-clipboard
 icon_color: pastel-pink
 ---