bans: change expires_at field to duration.

Changes:

* Change the `expires_at` field to `duration`.
* Make moderators choose from a fixed set of standard ban lengths,
  instead of allowing arbitrary ban lengths.
* List `duration` in seconds in the /bans.json API.
* Dump bans to BigQuery.

Note that some old bans have a negative duration. This is because their
expiration date was before their creation date, which is because in 2013
bans were migrated to Danbooru 2 and the original ban creation dates
were lost.

app/views/bans/_form.html.erb

@@ -1,6 +1,9 @@
 <%= edit_form_for(ban) do |f| %>
-  <%= f.input :user_name, :as => :string, :input_html => { data: { autocomplete: "user" } } %>
-  <%= f.input :duration, :hint => "in days" %>
+  <% if ban.new_record? %>
+    <%= f.input :user_name, as: :string, input_html: { "data-autocomplete": "user" } %>
+  <% end %>
+
+  <%= f.input :duration, collection: [["1 day", 1.day.iso8601], ["3 days", 3.days.iso8601], ["1 week", 1.week.iso8601], ["1 month", 1.month.iso8601], ["3 months", 3.months.iso8601], ["6 months", 6.months.iso8601], ["1 year", 1.year.iso8601], ["forever", 100.years.iso8601]] %>
   <%= f.input :reason %>
-  <%= f.button :submit, :value => "Ban" %>
+  <%= f.button :submit %>
 <% end %>