Support names starting with a digit or _ for projects and users

This is what’s actually allowed when creating a user or a project in gitlab.

CHANGELOG

@@ -23,6 +23,7 @@ v 7.8.1
   - Fix urls for the issues when relative url was enabled
   - Add Bitbucket omniauth provider.
   - Add Bitbucket importer.
+  - Support referencing issues to a project whose name starts with a digit
 
 v 7.8.0
   - Fix access control and protection against XSS for note attachments and other uploads.

lib/gitlab/markdown.rb

@@ -121,7 +121,7 @@ module Gitlab
       text
     end
 
-    NAME_STR = '[a-zA-Z][a-zA-Z0-9_\-\.]*'
+    NAME_STR = '[a-zA-Z0-9_][a-zA-Z0-9_\-\.]*'
     PROJ_STR = "(?<project>#{NAME_STR}/#{NAME_STR})"
 
     REFERENCE_PATTERN = %r{