Main::PSGI::View: implement response_timeline_list() method. Pass the tests.
t/HTML.t: add tests for timeline list page (root path)
t/Main_PSGI_View.t: set timeline_list_pager_entry_max config param before test, so that tests for response_timeline_list() succeed even if default setting of that variable changes.
Main: set default value of timeline_list_per_page.
Util: write specification of future_of() function. Add t/Util_future.t for its tests.