Insert MD links when autocompleting in MD mode

These will appear decorated because they are inserted as entities. It was necessary to modify pills to have an explicit linkText that is derived from the `href` being pillified (and is thus no longer the inserted completion but rather the display name (or user ID) or room alias.

src/autocomplete/UserProvider.js

@@ -56,13 +56,7 @@ export default class UserProvider extends AutocompleteProvider {
                 return {
                     completion: displayName,
                     suffix: range.start === 0 ? ': ' : ' ',
-                    entity: {
-                        type: 'LINK',
-                        mutability: 'IMMUTABLE',
-                        data: {
-                            url: 'https://matrix.to/#/' + user.userId,
-                        },
-                    },
+                    href: 'https://matrix.to/#/' + user.userId,
                     component: (
                         <PillCompletion
                             initialComponent={<MemberAvatar member={user} width={24} height={24}/>}