Add tags to terms index: Phase 1

[blchelle]

Description

Add unique keywords from the tags field to the indexed terms field

Example

Title: "I am the best at SQL" -> ["the", "best", "sql"]
Body: "But only SQLite" -> ["but", "only", "sqlite"]
Tags: "" -> ["sql", "sqlite", "db", "relational"]

Terms: ["the", "best", "sql", "but", "only", "sqlite", "db", "relational"]