Server Settings: Customize instance name and add contact URL (#5718)

* Server Settings: Customize instance name and add contact URL

- The custom instance name would improve #5563
- Added contact URL closes #4806

* Fix custom logo display

BTCPayServer/Models/ServerViewModels/BrandingViewModel.cs

@@ -0,0 +1,40 @@
+using System.ComponentModel.DataAnnotations;
+using BTCPayServer.Services;
+using Microsoft.AspNetCore.Http;
+using Newtonsoft.Json;
+
+namespace BTCPayServer.Models.ServerViewModels;
+
+public class BrandingViewModel
+{
+    // Server
+    [Display(Name = "Server Name")]
+    public string ServerName { get; set; }
+
+    [Display(Name = "Contact URL")]
+    public string ContactUrl { get; set; }
+
+    // Theme
+    [Display(Name = "Use custom theme")]
+    public bool CustomTheme { get; set; }
+
+    [Display(Name = "Custom Theme Extension Type")]
+    public ThemeExtension CustomThemeExtension { get; set; }
+
+    [JsonProperty(DefaultValueHandling = DefaultValueHandling.Populate)]
+    [MaxLength(500)]
+    [Display(Name = "Custom Theme CSS URL")]
+    public string CustomThemeCssUri { get; set; }
+
+    [Display(Name = "Custom Theme File")]
+    [JsonIgnore]
+    public IFormFile CustomThemeFile { get; set; }
+
+    public string CustomThemeFileId { get; set; }
+
+    [Display(Name = "Logo")]
+    [JsonIgnore]
+    public IFormFile LogoFile { get; set; }
+
+    public string LogoFileId { get; set; }
+}