Merge branch 'ordering_exchange_external' into 'develop'

Ordering for the exchange channels section (external) added

See merge request !345

external/exchange-channels/asperaweb/asperaweb.md

 ---
+order: 300
 layout: page
 permalink: /external/exchange-channels/asperaweb/
 shortcut: exchange-channels:asperaweb

external/exchange-channels/calendar/exchange-calendar.md

 ---
+order: 200
 layout: page
 permalink: /external/exchange-channels/calendar/
 shortcut: exchange-channels:calendar

external/exchange-channels/owncloud/owncloud.md

 ---
+order: 400
 layout: page
 permalink: /external/exchange-channels/owncloud/
 shortcut: exchange-channels:owncloud