[#119] Refactor `set-field` in CLI and WebAPI

[sancho20021]

Description

Problem: currently set-field is used both for changing visibility and setting new fields. This results in additional handling of the situation where both field contents and visibility option are missing.

Solution: require contents for setting field and allow changing visibility for convenience. Add set-field-visibility command in CLI and route in WebAPI which modifies only visibility.

Related issue(s)

Fixes #119

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Related changes (conditional)

• Tests

  • [x] If I added new functionality, I added tests covering it.
  • [x] If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.

• Documentation

  • [x] I checked whether I should update the docs and did so if necessary:
  • README
  • Haddock

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[sancho20021]

We've added new cmd. Seems we have to add it to README

True. Fixed. And also documentation about status codes