Unescape text before add mention, this commit solve #4776

[markdownify_helper](https://github.com/diaspora/diaspora/blob/develop/app/helpers/markdownify_helper.rb#L42) send to Diaspora::Mentionable.format method escaped string, for example if username is `@{Eve> E; eve@mail.com}` then escaped string is `@{Eve&gt; E; eve@mail.com}` and regular expression Diaspora::Mentionable::REGEX not work correct because `@{Eve&gt; E; eve@mail.com}` the character `;` is in wrong place
issue: #4919