Escape mentions before parsing message with markdown

Usernames that contained underscores were parsed by markdown first. This
broke the diaspora IDs and also added weird html at places where it
wasn't needed. Escaping them before sending the message through the
markdown parser fixes this issue.

As underscores are the only allowed character that can be used for
markdown that is also allowed inside a diaspora ID, this escaping can be
kept pretty simple.

This only fixes it for the mobile UI at the moment, for the desktop UI
it's probably better to fix it in markdown-it.

Related to #7975

lib/diaspora/mentionable.rb

@@ -71,6 +71,16 @@ module Diaspora::Mentionable
     }
   end
 
+  # Escapes special chars in mentions to not be parsed as markdown
+  #
+  # @param [String] text containing mentions
+  # @return [String] escaped message
+  def self.escape_for_markdown(msg_text)
+    msg_text.to_s.gsub(REGEX) {|match_str|
+      match_str.gsub("_", "\\_")
+    }
+  end
+
   private_class_method def self.find_or_fetch_person_by_identifier(identifier)
     Person.find_or_fetch_by_identifier(identifier) if Validation::Rule::DiasporaId.new.valid_value?(identifier)
   rescue DiasporaFederation::Discovery::DiscoveryError

lib/diaspora/message_renderer.rb

@@ -71,6 +71,10 @@ module Diaspora
         end
       end
 
+      def escape_mentions_for_markdown
+        @message = Diaspora::Mentionable.escape_for_markdown(message)
+      end
+
       def render_mentions
         unless options[:disable_hovercards] || options[:mentioned_people].empty?
           @message = Diaspora::Mentionable.format message, options[:mentioned_people]
@@ -210,6 +214,7 @@ module Diaspora
         normalize
         diaspora_links
         camo_urls if AppConfig.privacy.camo.proxy_markdown_images?
+        escape_mentions_for_markdown
         markdownify
         render_mentions
         render_tags