Via Tender: Clean up unused plot keywords

Assigned to Tender discussion #1275. As reported in Tender:

I have a few questions regarding plot keywords.

1. When adding plot keywords a list of plot keywords appears as I type. Why are there so many identical keywords on the list?

2. Keywords are subjective. Sometimes there are different keywords that mean the same thing, for example: "based on book" and "based on novel" or "mental illness" and "mental disorder". Could there be a way to merge keywords that mean the same thing?

3. Keywords that are not used for any movie's plot keywords, can they be purged from the database? Such as this huge mistake someone made: " sister sister relationship * rebellious daughter * boyfriend girlfriend relationship * mother daughter relationship * tragedy * husband wife relationship * cancer * sick child * family crisis * sadness * hospital * family relationships * funeral * terminal illness * punctuation in title * father daughter relationship * apostrophe in title * based on novel"