Convert "Page Size" field in Maniphest Advanced Search to PhabricatorSearchIntField

Summary:
Convert the "Page Size" field from PhabricatorSearchTextField to PhabricatorSearchIntField as its input should be an integer value (the number of tasks to display in the query results).

Closes T15723

Test Plan:

• Go to /maniphest/query/advanced/
• enter random values in the Page Size field and run random Maniphest queries (example values: 13, 0, -13, foobar, foo13bar, -foo1313, 13.5, -13.5, 13,5, -13,5, -, -0).

Reviewers: O1 Blessed Committers, 20after4

Reviewed By: O1 Blessed Committers, 20after4

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15723

Differential Revision: https://we.phorge.it/D25527