Allow only positive and negative integer values as input in a PhabricatorSearchIntField.

Use getStr() instead of getInt() to allow differentiation in validateControlValue() between a literal value 0 versus 0 only returned by PHP casting a random string value to an int.
The validateControlValue() function does not use is_numeric($value) as this would also accept float values, and does not use is_int($value) as that is impossible due to the previous getStr().
Generally, the code is similar to PhabricatorSearchDateField.php.

See T15700

• Make sure that rPfbe07fbeefcadea3abdb4dc3e4d4558c2b91ada9 is included
• Go to /maniphest/query/advanced/ and enter example values into the Page Size field and click the Search button - get Integer value for "Page Size" can not be parsed. now for invalid example values.
• In /src/applications/project/query/PhabricatorProjectSearchEngine.php, change the two only uses of PhabricatorSearchIntField to ->setIsHidden(false). Then go to /project/query/advanced/ and enter example values into the Maximum Depth and Maximum Depth fields and click the Search button.
• Example values: 13, 0, -13, foobar, foo13bar, -foo1313, 13.5, -13.5, 13,5, -13,5, -, -0 (the latter is still accepted as 0).
• API call /conduit/method/project.query/ does not offer passing depth values to test.
• API call for /conduit/method/maniphest.search/ and entering a non-integer value as limit triggers an error but this code path does not call PhabricatorSearchIntField.php at all, so this is unrelated and a separate issue.