nmbug-status: Wrap query phrases in parentheses when and-ing together

nmbug-status: Wrap query phrases in parentheses when and-ing together

For example:

  "query": ["tag:a", "tag:b or tag:c"]

is now converted to:

  ( tag:a ) and ( tag:b or tag:c )

instead of the old:

  tag:a and tag:b or tag:c

This helps us avoid confusion due to Xapian's higher-precedence AND
[1], where the old query would be interpreted as:

  ( tag:a and tag:b ) or tag:c

[1]: http://xapian.org/docs/queryparser.html