User space

It will be really nice to let every user have their own space.
In order to see their notes, activity or settings.
But this will related to email binding and username binding.
The ideal url for the user will be: `https://hackmd.io/@jackycute`, which means use @ to differ users (like Medium dose).

The issue names no files, tests, or entry points. First map the existing email and username binding, note activity, settings, and URL routing flows, then clarify the account and profile scope before implementation. Done should include a defined @username URL and user-specific notes, activity, and settings behavior.

Written by the indexing model from the issue text.