[web/httpcache] fix Last-Modified generation time.mktime takes a time tuple in *local* time. We have an UTC datetime coming from the last_modified method, which we were interpreting as local time, leading to hilarity. Use calendar.timegm instead. Add tests for Last-Modified generation of the 'download' and 'manage' views.

[forms] Fix edition of TZDatetime attributes Parse the submitted web form values into a timezone-aware datetime (by default, UTC) when such an attribute is edited. This is done using an automatically-selected (by `web.formfield.guess_field`) field named `TZDatetimeField`. Closes #12666598.