Replace Convos::Oembed with Mojolicious::Plugin::LinkEmbedder

Using Mojolicious::Plugin::LinkEmbedder should in theory enable us to embed more content from twitter, github, ...

But I have some questions:
* Is LinkEmbedder safe? I mean, can we include alien links that can do cross site scripting? ..in an obvious way?
* Do we need to do something with the carton config, now that I've added another dependency?