add: custom multisub pages similar to 4chan's directories

[plebeius-eth]

Board names on 4chan are called "directories", such as /biz/, /pol/, etc. We should probably group the addresses in our default list by directory, achieving a board list more similar to 4chan's. For example, all crypto-related subs could be grouped under /biz/.

It should still be possible to access one subplebbit directly, of course. But the list at the top of the board pages should be concise like 4chan's, with one directory per subject/category.

https://t.me/plebchanreact/7780

[plebeius-eth]

https://t.me/plebchanreact/8779