Use UUIDs for Bookmark unique ID

Use UUIDs for Bookmark unique ID

I was trying to generate URIs for Bookmarks based on (profile, crc32(url), created).

I failed at that. CRC32s are unsigned ints, and our schema code didn't like that.

On top of that, my code to encode and restore created timestamps was problematic.

So, I switched back to using a meaningless unique ID for Bookmarks.

One way to do this would be to use an auto-incrementing integer ID. However, we've been
kind of crabbed out a few times for exposing auto-incrementing integer IDs as URIs, so
I thought maybe using a random UUID would be a better way to do it.

So, this patch sets random UUIDs for URIs of bookmarks.