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