Fix: remove emails placeholder (coscine/issues#709)

coscine/issues#709

src/Migrator/Migrations/Migration202003192117ORCiDEmailAddress.cs

+using FluentMigrator;
+namespace Coscine.Database.Migration.Migrations
+{
+    //yyyymmddhhmm
+    [Migration(202003192117)]
+    public class Migration202003192117ORCiDEmailAddress : FluentMigrator.Migration
+    {
+        public override void Down()
+        {
+            Alter.Column("EMailAddress").OnTable("Users").AsString(200).NotNullable();
+        }
+
+        public override void Up()
+        { 
+            Alter.Column("EMailAddress").OnTable("Users").AsString(200).Nullable();
+        }
+    }
+
+}