User search REST endpoint does not trim input or apply minimum input length restrictions

Issue Summary

By issuing a user search with multiple spaces it is possible to put high load on the database server.

Steps to Reproduce

Submit a REST request to

/rest/api/latest/projects/{PROJECT_KEY}/repos/{REPO_SLUG}/permissions/users/none?start=0&filter={MY_FILTER}

or 

/rest/ui/latest/projects/{PROJECT_KEY}/repos/{REPO_SLUG}/reviewer-groups?avatarSize=32&permission=LICENSED_USER&permission.1=REPO_READ&permission.1.repositoryId=1&start=0&filter={MY_FILTER}

 where MY_FILTER is short, or padded by whitespace

Expected Results

No requests made to the DB as the filter is very short.

Actual Results

The request is made to the DB, and the filter is not trimmed.

This was split from https://jira.atlassian.com/browse/BSERV-12650 and represents fixing the backend component of that bug