Escape search terms before passing to Regexp

## What does this MR do?

Use `Regexp.escape` to escape the search terms before passing them to `Regexp.new`.

## Why was this MR needed?

- evaluated regular expressions in search terms lead to unexpected result
- unbalanced parentheses in search term lead to server error 

## TODO

- [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added

## What are the relevant issue numbers?

fixes #14360

See merge request !6241