refactor importer

[obfusk]

• part 1: refactor the Catima importer refactor importer part 1 #1422
• part 2: refactor the other importers refactor importer part 2 #1434
• part 3: use background threads
• part 4: create & use a ConflictResolver that allows the user to choose how to resolve conflicts

---

• see merge imports #1394 for discussion on conflict resolution
• migrate to timestamp IDs to avoid conflicts and renumbering
• handle duplicates with different IDs (requires looping)
• handle partial duplicates (let the user choose which fields to compare)?

---

Related: #512, #513.

NB: migrating to group IDs requires changes to handling importing groups and mappings.

---

• refactor stocard importer StocardImporter: refactor #1443