Add the ability to organise subscriptions into nested folders

**Is your feature request related to a usage problem (not a bug)? Please describe.**
I use PocketTube browser extension to organise my YouTube subscriptions currently and it allows for nested folders to sort all my subscriptions into. I'd like that feature to be available for FreeTube instead.

**Describe the solution you'd like to see implemented**
Basically just copy the PocketTube UI.

**Describe alternatives you've considered**
Only alternative is using PocketTube itself but I'd like to use FreeTube for obvious reasons.

**Screenshots**
N/A

**Additional context**
N/A