Use timezone not offset for user activity, fix heat map
    
    My previous attempt (in f99ac7c) still had two clear problems.
    Using the *current* offset from UTC for the local timezone
    isn't really good enough: for timezones that have daylight
    savings, for instance, it'll be wrong for events that happened
    in the other state (so, events that happened during daylight
    savings when the query is run *outside* of daylight savings,
    for instance).
    
    Also, the heatmap could still be wrong, because while we now
    always had the right target date in mind, we were not smart
    enough about making sure we fed cal-heatmap a timestamp that
    definitely fell on that date in the local timezone.
    
    This should fix both problems. Unfortunately, we need a new JS
    library to do it. Getting the actual timezone (as opposed to
    the offset) is a bit tricky; it is possible to get it from many
    newer browsers via the Internationalization API, but some still
    do not support this, so best practice is to use a library which
    takes that value if possible, but otherwise tries to figure out
    the timezone by requesting the offset at various points in time
    and inferring from the reported values.
    
    We change `PagureLog.date_offset()` from the previous attempt to
    `PagureLog.date_tz()`, expecting a timezone name (Olson format),
    and use it much as before. To solve the heatmap issue, we try
    to get 12:00 on the target date in the local timezone, and
    convert that to a timestamp.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>
    
    Merges https://pagure.io/pagure/pull-request/3030
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>