User account workspace assignment (#751).

The "System" workspace is introduced.

When creating an User Account:
- a private workspace is created for the user.
- the User Account and the Password are assigned to the user's private workspace.
- the Username is assigned to the "System" workspace.

NOTE: login is broken with this commit.
Checking the credentials must perform with special privilegs (inside the Core).

BREAKING CHANGES

Workspaces API:

1 dropped method:
{{{
Topic getDefaultWorkspace()
}}}

1 new method:
{{{
Topic getWorkspace(String uri)
}}}
Returns a workspace by URI.
If no workspace exists for the given URI a runtime exception is thrown.

3 new constants in `WorkspacesService` interface:
{{{
String DEEPAMEHTA_WORKSPACE_NAME
String DEEPAMEHTA_WORKSPACE_URI
SharingMode DEEPAMEHTA_WORKSPACE_SHARING_MODE
}}}

To get the "DeepaMehta" workspace:
{{{
Topic workspace = wsService.getWorkspace(WorkspacesService.DEEPAMEHTA_WORKSPACE_URI)
}}}

See #751.