Allow initialization of the main DICOM server via URL

[fedorov]

Follow up on https://github.com/ImagingDataCommons/slim/issues/5

Currently, it is possible to select at runtime main server via UI, but not via URL. The latter is implemented in OHIF and would be very convenient in some situations. The main use case for this feature is when the URLs are pre-generated in a query or by code, and, for example, need to be provided in a user-facing dashboard.