LP#1721807: fix webstaff report templates that have might_have and has_many joins

LP#1721807: fix webstaff report templates that have might_have and has_many joins

This patch fixes a bug where using a virtual field in the web staff
report template editor to join another table would result in a syntax
error in the generated SQL.

To test
-------
[1] Apply the patch and verify that the regression test in the previous
    commit passes.
[2] Create new report template in the web staff client that include
    virtual fields and joins and verify that they work. One example
    would be a report that uses ILS User as the base source and joins
    in the patron notes virtual field.
[3] If available, clone a report template originally created in the web
    staff client that had previously failed, then save it. The new
    report should work.

Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>