models/IssueFilter: fix converting filter to user_query

_to_user_query method doesn't escape single quote characters for regular expressions (stdout/stderr/dmesg) but we should do it because these expressions are wrapped in singel quote characters. Not escaping them results it not working user_queries.

For example let's say we have filter with such stderr regexp: test's log. It's correct expression. _to_user_query method would produce following user query: stderr ~= 'test's log'. It results in error, expecitng OR/AND keyword instead of s log' string.