Improve update channel layout to be a popup menu

* now the entries are in a popup menu
* if a non-default channel is used the entry will be at the bottom
* channel descriptions are embeded into the menu
* use nextcloud-vue componenets for the channel selector
* update nextcloud-vue dependency from 0.6.1 to 0.6.3
* fixes #13806

Signed-off-by: Morris Jobke <hey@morrisjobke.de>