Preserve duration when editing 1w ban

[ErikHumphrey]

When you edit a 1 week ban, often to edit the ban reason, the duration dropdown is always reset to 1 day. Editing the ban should default to the current ban duration so you don't mistakenly reduce the ban length to 1 day.

[nottalulah]

This only seems to happen for one-week bans, because they get converted to "7 days" and the iso8601 entries are different (P7D vs P1W). The other lengths aren't reset to 1 day.