Improved search

Main changes

• Refactored searching for users and topics:
  • Two types of queries: transient fnd.public and long lived: fnd.private. Transient queries are not saved on the server and not shared between user's devices/clients. Long lived queries are persisted at the server and shared between devices.
  • Queries are strings now (as opposed as string arrays before). A formal query language is introduced.
  • An option added for designating some tags as restricted. It's needed for managing organizations within a server.
• Structured private field in topics and users. The private used to contain a string. Now it holds an object with private.comment holding what was the content of private.
• touched field is added to report last message time stamp. It's needed for sorting topics by the time of last activity.
• Graphic assets updated with the new logo
• Bug fixes