new word replacements table should not have unique index

in v4.4.x the word replacements utility was moved to a dedicated table. that table has a unique index on the find_word column (in 4.4.1 the index was changed to be on find_word and domain_id).

that creates a problem, because the word replacement could be case sensitive, but the mysql table is case insensitive.

for example, in the context of cases, you might want to replace "client" with "constituent" and "Client" with "Constituent" – thus requiring two replacement entries. with the unique index, you can't do that.