feat(applications): Add icon to applications (#681)

* the ApplicationAPI now returns a new parameter called `icon` that contains the url to the icon
* the icon is served by the servlet `../API/applicationIcon/<id>?t=<lastUpdateDate>`

example of application API response:
```
{
  updatedBy: "-1",
  displayName: "Admin app",
  lastUpdateDate: "1605275321167",
  description: "",
  themeId: "912",
  homePageId: "1001",
  creationDate: "1605275320903",
  version: "0.0.3",
  layoutId: "805",
  token: "admin",
  createdBy: "-1",
  profileId: "2",
  iconPath: "../API/applicationIcon/901?t=1605275321167"
  id: "901",
  state: "ACTIVATED",
  iconPath: ""
}
```

Relates to [RUNTIME-114](https://bonitasoft.atlassian.net/browse/RUNTIME-114)