Improve performance of similar alleles endpoint

Description

Performance was poor on the original implementation, discovered when tested in a production-like environment.

Rewrote to avoid SQL queries and loading alleles in a loop. Now everything is done in a single SQL query, and a single call to AlleleDataLoader.

Related issues

Notes to review (code/docs/QA)

Tests

General

• Tests have been added that prove my fix is effective or that my feature works
• Related tests have been modified/removed

Hypothesis testing:

• Soak testing has been done
• Distribution between positive / negative cases has been checked

Database

• Includes changes to database schema
• Includes necessary database migrations

Configuration

• Includes changes to configuration
• Includes configuration migration instructions in documentation

Merge checklist

• Self-review of code has been performed.
• Feature review and validation against specification has been performed (if applicable). Apply label: QAdone
• Need for documentation has been evaluated and, if necessary, updated. Apply label: docsdone
• Code and implementation has been reviewed by other core developer (including any changes based on initial review). Apply label: code reviewdone

Closes #278 (closed)