User range recommendations; Fixes #177

[joe4dev]

Add user range support as described in #177 Maintains the existing behavior when triggering single recommendations.

Range generation now separates constructing (i.e., generating the MiniZinc model) from evaluation. See 17a99c0 for more detail.

• [x] Implement error handling (e.g., recommendation for non-root ingredients, catch exceptions)
• [x] Remove legacy way of generating deployment recommendations (including seeds!)
• [x] Test better: particularly check whether the http://localhost:3000/ingredients/:id/recommendations_completed works consistently; check concurrent execution with multiple workers

Dependency: Already rebased on PR #176

[dschoeni]

I started implementing the corresponding frontend parts.

GET: /recommendations_completed always returns false though, even if there are no recommendations scheduled anymore. Triggering ranges of recommendations works well however ;)

[joe4dev]

Works for me with the latest version and also with some legacy db dumps I migrated for testing purpose. Also works for @dschoeni now (at least after db reset)

Should be ok but might need some more testing with concurrent executions (as stated in the PR tasks)

[joe4dev]

Looks ready to merge for me now