[CT-1661] Quotes usernames with special characters

[qmg-karan]

resolves #156

Problem

This PR solves the issues where dbt tries to apply grant-related SQL queries against users that have special characters in their names and fails.

Solution

Quotes are added around users that hold special characters. So far, this just includes a few special characters that people have raised issues for across several dbt repos ("." in #156, "-" in #55, and "@" in my case.

Please suggest how this can be tested, I'm not entirely sure about this because the unit-tests seem to pass after I've made this change. Not sure where the tests should be added for this.

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development, and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc.) or this PR has already received feedback and approval from Product or DX

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[qmg-karan]

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Signed now