Create "centralized" profile storage

**Is your feature request related to a problem? Please describe.**

ZeroNet has many communication sites. Although you can use most of them with ZeroID, many of them also require additional profile data, such as icon, name, description and other. Some of that data is unique for each site, but some of them could be shared.

In order to set or change some data, the user needs to change them on each site and multiple times. If not, some of the data that should be the same across multiple sites (like the name) could be different and this could be confusing.

**Describe the solution you'd like**

There needs to be some kind of "centralized" profile storage for data that would be accessed and used across multiple ZeroNet sites.

That storage would have to store multiple kinds of data. General data (icon, username, name, description...) would be stored in general namespaces. Some site-specific data would be stored in site namespace and other site-specific data (posts...) should be stored in the site as currently. There would also need to be support for multiple accounts and multiple IDs for each user and relations between them. Private encrypted data should also be supported.

Here I don't mean a centralized site to store all data but site completely decentralized and managed by the user. The possible implementation of this would be that each user would have its own ZeroNet site dedicated to profiling storage. Other sites would them somehow know the address of that site (maybe with additional site index site) and query data from it.

**Additional context**

Similar projects to this are [3Box](https://3box.io/) for Ethereum and [Gravatar](https://gravatar.com/) for clearnet.