Named notes-only rooms per space ("yourself" room for each space)

[turt2live]

an extension of #737 - it'd be nice to have at least a per-space notepad to write things down without needing to leave Element. A simple implementation would be to offer an "Add only for you" option on spaces that the user can then rename. A more sophisticated implementation might just be a giant wysiwyg composer backed by a room or account data or something.

Current workaround I'm trialing is creating a room, inviting my backup account (which happens to be a member of the space), then using /converttodm to make it a "DM" that's then automatically visible in the space. The create->invite->convert workflow is to avoid pseudo-canonical DMs from kicking in and redirecting me to an existing DM room I have.