Refactor Relation Fields and Global Queries

[dvchristianbors]

Closes #417 Depends on https://github.com/datavisyn/reprovisyn/pull/446

Developer Checklist (Definition of Done)

• [x] Descriptive title for this pull request provided (will be used for release notes later)
• [x] All acceptance criteria from the issue are met
• [x] Branch is up-to-date with the branch to be merged with, i.e., develop
• [x] Code is cleaned up and formatted
• [ ] Code documentation written
• [ ] Unit tests written
• [x] Tested in Chrome
• [ ] Tested in Firefox
• [x] Build is successful
• [x] Summary of changes written
• [ ] Wiki documentation written
• [x] Assign at least one reviewer
• [x] Assign at least one assignee
• [x] Add type label (e.g., bug, enhancement) to this pull request
• [ ] Add next version label (e.g., next-version: minor) to this PR following semver

Summary of changes

• Omit source to target, relation direction directly obtained from the backend API
• Refactor Global Queries
  • moved fields and properties from separate object into entity
  • Avoid sql-injectable code

Screenshots

UI remains unchanged.