refactor: migrate to diesel-rs for query construction

[drahnr]

A example of using diesel.rs as an ORM for a single query.

Learnings

• Caveats

  • types in models.rs need to align precisely with the schema generated from the database in schema.rs - otherwise one gets lengthy errors - but at least at compile time
  • filters still require type conversions to the native DB representation
  • requires an intermediate type to represent a row (it's good, but sometimes might get in the way
  • additional complexity

• Upsides

  • More structured / a little less boilerplate
  • No inline SQL (if desired)
  • Drop in for postgres possible

Outstanding

• Understand how to load records partially
• Migration integration - deferred

Closes #921

[drahnr]

Let's have a call on Monday post our regular calls and sync up then @sergerad @Mirko-von-Leipzig

[Mirko-von-Leipzig]

Couple of questions but I think we should proceed with this before we damage it (and its authors) health any more :D