Add auto-registration mechanism

- Add User registration mechanism when adding auth provider id to claims, if no User Id corresponds to the auth provider Id
- Swallow unique constraint violation for unique auth provider id on user table, in case of duplicate requests
- Add Serilog logging
- Add no bookmarks and no tags message when none is found on Bookmark List page

YABA.Service/UserService.cs

@@ -1,4 +1,8 @@
 using AutoMapper;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.Logging;
+using Npgsql;
+using System;
 using System.Linq;
 using System.Threading.Tasks;
 using YABA.Common.DTOs;
@@ -13,12 +17,14 @@ namespace YABA.Service
         private readonly YABAReadOnlyContext _roContext;
         private readonly YABAReadWriteContext _context;
         private readonly IMapper _mapper;
+        private readonly ILogger<UserService> _logger;
 
-        public UserService (YABAReadOnlyContext roContext, YABAReadWriteContext context, IMapper mapper)
+        public UserService (YABAReadOnlyContext roContext, YABAReadWriteContext context, IMapper mapper, ILogger<UserService> logger)
         {
             _roContext = roContext;
             _context = context;
             _mapper = mapper;
+            _logger = logger;
         }
 
         public bool IsUserRegistered(string authProviderId)
@@ -28,19 +34,40 @@ namespace YABA.Service
 
         public async Task<UserDTO> RegisterUser(string authProviderId)
         {
-            if(IsUserRegistered(authProviderId))
+            try
             {
-                var user = _roContext.Users.FirstOrDefault(x => x.Auth0Id == authProviderId);
-                return _mapper.Map<UserDTO>(user);
+                if (!IsUserRegistered(authProviderId))
+                {
+                    var userToRegister = new User
+                    {
+                        Auth0Id = authProviderId
+                    };
+
+                    var registedUser = _context.Users.Add(userToRegister);
+                    await _context.SaveChangesAsync();
+                }
+            } 
+            catch (Exception ex)
+            {
+                if(ex.InnerException is PostgresException &&
+                    ((PostgresException)ex.InnerException).Code == "23505")
+                {
+                    var postgresException = (PostgresException)ex.InnerException;
+                    _logger.LogWarning("Swallowing constraint violation: {@ConstraintName} for {@AuthProviderId}", postgresException.ConstraintName, authProviderId);
+                }
+                else
+                {
+                    throw ex;
+                }
             }
 
-            var userToRegister = new User
-            {
-                Auth0Id = authProviderId
-            };
+            return await Get(authProviderId);
+        }
 
-            var registedUser = _context.Users.Add(userToRegister);
-            return await _context.SaveChangesAsync() > 0 ? _mapper.Map<UserDTO>(registedUser.Entity) : null;
+        public async Task<UserDTO> Get(string authProviderId)
+        {
+            var user = await _roContext.Users.FirstOrDefaultAsync(x => x.Auth0Id == authProviderId);
+            return _mapper.Map<UserDTO>(user);
         }
 
         public int GetUserId(string authProviderId)