Enable custom panel by default and have default content

For now, it just says "This is a Pleroma instance". I think it'd be useful to have it link to the PleromaFE / MastoFE (both, anticipating that the custom panel will also be shown on mastofe: mastofe#12 (closed)), as well as a link to pleroma.social.

This way we can implement #141 (closed), while giving admins control on the custom panel and whether to show or hide more links.