Prettier Loading

[lowtorola]

• Adds a loading prop to section card for nicer handling of loading data which its content depends on
• Adds some checks to certain pages so we redirect on data failure instead of blocking with an ugly loading interstitial
• Changes up our section card header to something that i think looks nicer (looking for feedback though)
• Adds preliminary content for bc25 (will need to create actual bc25java and bc25python episodes/content saturday?)

Closes #789 🚀