Adapt WebApi to latest changes of the registry interface

[johannesduesing]

Reason for this PR
The interface of the instance registry has been changed lately. The reason for this was to better support the requirements of the Delphi-Management state management. The relevant change is:

• The attributes linksTo: List[InstanceLink] and linksFrom: List[InstanceLink] have been added to the Instance class
  This PR adapts the WebApi to this change. The same has been done for the WebApp.

Changes

• Added file InstanceLink.scala containing the JSON support
• Added new attributes to Instance

[codecov-io]

Codecov Report

Merging #36 into develop will decrease coverage by 0.04%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           develop    #36      +/-   ##
=========================================
- Coverage     1.44%   1.4%   -0.05%     
=========================================
  Files           17     18       +1     
  Lines          414    427      +13     
  Branches        31     31              
=========================================
  Hits             6      6              
- Misses         408    421      +13

Impacted Files	Coverage Δ
...la/de/upb/cs/swt/delphi/webapi/Configuration.scala	0% <ø> (ø)	⬆️
...s/swt/delphi/instancemanagement/InstanceLink.scala	0% <0%> (ø)
...pb/cs/swt/delphi/instancemanagement/Instance.scala	0% <0%> (ø)	⬆️
...t/delphi/instancemanagement/InstanceRegistry.scala	0% <0%> (ø)	⬆️

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