Product/202 user invitation

coscine/issues#202

src/Database/Models/InvitationModel.cs

+using Coscine.Database.DataModel;
+using Microsoft.EntityFrameworkCore;
+using System;
+using System.Linq.Expressions;
+
+namespace Coscine.Database.Models
+{
+    public class InvitationModel : DatabaseModel<Invitation>
+    {
+        public override Expression<Func<Invitation, Guid>> GetIdFromObject()
+        {
+            return databaseObject => databaseObject.Id;
+        }
+
+        public override DbSet<Invitation> GetITableFromDatabase(CoscineDB db)
+        {
+            return db.Invitations;
+        }
+
+        public override void SetObjectId(Invitation databaseObject, Guid id)
+        {
+            databaseObject.Id = id;
+        }
+
+        public Invitation GetByToken(Guid token)
+        {
+            return GetWhere(x => x.Token == token);
+        }
+
+        public Guid CreateInvitation(Guid project, Guid issuer, Guid roleId, string inviteeEmail)
+        {
+            var token = Guid.NewGuid();
+
+            Insert(
+                new Invitation
+                {
+                    Id = Guid.NewGuid(),
+                    Project = project,
+                    Issuer = issuer,
+                    Role = roleId,
+                    InviteeEmail = inviteeEmail,
+                    Expiration = DateTime.UtcNow.AddDays(7),
+                    Token = token,
+
+                }
+            );
+
+            return token;
+        }
+    }
+}