Allow mentions

As more I work I need to plan how mentions will work. So how I am planning:

A ckeditor5 mentions plugin to assist while writing, so it suggests accounts:

• Suggest Following and followers (it will be great to store account preferredname@domain somewhere in the actor entity when following or receiving follower...) to be able to use @preferredusername@domain instead of the url).
• If one writes @bob@example.org it should try to resolve (as fast as it can so it needs some caching) and suggests the account if resolved. If well resolved it will transform to <span class=\"h-card\"><a href=\"https://example.com/@bob\" class=\"u-url mention\">@<span>bob</span>@example.com</a></span> (some implementations do not prepend domain when using stored actors but I thing it's better to add it).

When submitting, a microformats2 parser could transform to mentions as specified in https://www.w3.org/TR/activitystreams-vocabulary/#microsyntaxes .

We are waiting for ckeditor5 feature request and https://github.com/ckeditor/ckeditor5/issues/12907 , and need to discuss how can we get the preferred username from the followers/following . So while not there, by now, I'll create a ckeditor5 plugin for link, to allow add the url of the user directly and it will surround with microformats and mention as mastodon does.