Forced LIMIT clause not being applied correctly when SQL has a trailing comment

[dflionis]

When a user submits a query that ends in a comment, Superset incorrectly appends its built-in LIMIT clause to that same last line which makes the LIMIT clause commented out as well.

Expected results

Ideally the limit would be added on a new line so that SQL comments cannot have any effect on it.

Actual results

Superset appends the LIMIT clause to the same line which is functionally equivalent to not adding a limit at all.

Screenshots

User submits the following query (with a default LIMIT of 500 in the UI set):

SELECT y, m, d, COUNT(*)
FROM some_table
GROUP BY y, m, d -- uh oh a trailing comment!

Superset submits to the database engine:

SELECT y, m, d, COUNT(*)
FROM some_table
GROUP BY y, m, d -- uh oh a trailing comment! LIMIT 500

How to reproduce the bug

Run a query that ends in a trailing comment, such as the example user submission in the Screenshots section above.

Environment

• superset version: 0.32.0rc2
• python version: 3.6
• node.js version: ???
• npm version: ???

Checklist

Make sure these boxes are checked before submitting your issue - thank you!

• I have checked the superset logs for python stacktraces and included it here as text if there are any.
• I have reproduced the issue with at least the latest released version of superset.
• I have checked the issue tracker for the same issue and I haven't found one similar.

Additional context

I'm not sure if its as simple as inserting a newline here but I'm having trouble getting the tests to pass locally so I didn't want to submit a PR that I'm not confident about.

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