✨(backend) search users

We need to search users by their email.
For that we will use the trigram similarity algorithm
provided by PostgreSQL. To use it we have to
activate the pg_trgm extension in postgres db.
To query the email we will use the query param
`q`.
We have another query param `document_id`, it is
necessary to exclude the users that have already
access to the document.

CHANGELOG.md

@@ -14,6 +14,7 @@ and this project adheres to
 - Update document (#68)
 - Remove document (#68)
 - (docker) dockerize dev frontend (#63)
+- (backend) list users with email filtering (#79)
 
 ## Changed