fix: id of content to be unique

[max-nextcloud]

content is already used as an id in the server template: https://github.com/nextcloud/server/blob/c10317f7f9f53a4de464915f754896eed7f1ee6c/core/templates/layout.user.php#L125

Make sure to have a unique id and to replace the correct dom element when mounting the vue app.

Fixes #474.

[max-nextcloud]

I replaced the div#content in the template with a comment so we don't have the same id on the template provided by the server. I hope this also makes it more clear that the content of the template will be overwritten anyway.