Friendly post urls

Rather than a format like piefed.social/post/long_number, posts how have a url that includes the community name and title in it.

Like this: https://piefed.social/c/animatedmusicvideos@sh.itjust.works/p/1382987/junior-senior-move-your-feet-official-music-video-hd

This improves readability, helps people differentiate between multiple links in a list and is better for SEO.

The old style of url still works too and there may be occasions when you get taken to a url like https://piefed.social/post/1382987 but we'll be working to remove those over time.