Adapt WebApp 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 WebApp to this change.

Changes

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

[codecov-io]

Codecov Report

Merging #24 into develop will not change coverage.
The diff coverage is 0%.

@@          Coverage Diff          @@
##           develop   #24   +/-   ##
=====================================
  Coverage        0%    0%           
=====================================
  Files           10    11    +1     
  Lines          234   248   +14     
  Branches        23    28    +5     
=====================================
- Misses         234   248   +14

Impacted Files	Coverage Δ
app/utils/Configuration.scala	0% <ø> (ø)	⬆️
app/utils/instancemanagement/InstanceLink.scala	0% <0%> (ø)
...pp/utils/instancemanagement/InstanceRegistry.scala	0% <0%> (ø)	⬆️
app/utils/instancemanagement/Instance.scala	0% <0%> (ø)	⬆️

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