Make internal properties unreadable and unwritable by users

[Syndesi]

Related to #178 .

Acceptance criteria:

• Users should not be able to directly write internal properties, i.e. those starting with _.
• Users should not be able to read internal properties; they should be excluded from search, element and collection responses.
• Where possible, search queries referencing internal properties should fail.
  • Elasticsearch Query DSL
  • Cypher Path Subset
  • Element Hydration