Ability to define/see preview URL for Theia tasks

[vparfonov]

Description

User should have ability during creation of a new task (command in Che terminology) to provide Preview URL using macros (see #8974).
After successful execution of a command this URL will be shown on UI

[azatsarynnyy]

A preview URL can be defined by adding the optional filed previewURL in a task definition:

{
    "type": "che",
    "label": "...",
    "command": "...",
    "target": {
        "machineName": "..."
    },
    "previewUrl": "${server.tomcat8}"
}

Variable substitution is supported.