Refactoring: Allow ViewsRazor extension to be used by plugins

Moves the `ViewsRazor` extension into Abstractions, so that it can be used by plugins.

Separated out of #2701, prerequisite for the LNbank plugin integration.

BTCPayServer.Abstractions/Extensions/ViewsRazor.cs

@@ -0,0 +1,92 @@
+using System;
+using System.Globalization;
+using Microsoft.AspNetCore.Html;
+using Microsoft.AspNetCore.Mvc.ViewFeatures;
+
+namespace BTCPayServer.Abstractions.Extensions
+{
+    public static class ViewsRazor
+    {
+        private const string ACTIVE_CATEGORY_KEY = "ActiveCategory";
+        private const string ACTIVE_PAGE_KEY = "ActivePage";
+
+        public static void SetActivePageAndTitle<T>(this ViewDataDictionary viewData, T activePage, string title = null, string mainTitle = null)
+            where T : IConvertible
+        {
+            // Browser Title
+            viewData["Title"] = title ?? activePage.ToString();
+            // Breadcrumb
+            viewData["MainTitle"] = mainTitle;
+            viewData["PageTitle"] = title;
+            // Navigation
+            viewData[ACTIVE_PAGE_KEY] = activePage;
+            SetActiveCategory(viewData, activePage.GetType());
+        }
+
+        public static void SetActiveCategory<T>(this ViewDataDictionary viewData, T activeCategory)
+        {
+            viewData[ACTIVE_CATEGORY_KEY] = activeCategory;
+        }
+
+        public static string IsActiveCategory<T>(this ViewDataDictionary viewData, T category)
+        {
+            if (!viewData.ContainsKey(ACTIVE_CATEGORY_KEY))
+            {
+                return null;
+            }
+            var activeCategory = (T)viewData[ACTIVE_CATEGORY_KEY];
+            return category.Equals(activeCategory) ? "active" : null;
+        }
+
+        public static string IsActivePage<T>(this ViewDataDictionary viewData, T page)
+            where T : IConvertible
+        {
+            if (!viewData.ContainsKey(ACTIVE_PAGE_KEY))
+            {
+                return null;
+            }
+            var activePage = (T)viewData[ACTIVE_PAGE_KEY];
+            return page.Equals(activePage) ? "active" : null;
+        }
+
+        public static HtmlString ToBrowserDate(this DateTimeOffset date)
+        {
+            var displayDate = date.ToString("o", CultureInfo.InvariantCulture);
+            return new HtmlString($"<span class='localizeDate'>{displayDate}</span>");
+        }
+
+        public static HtmlString ToBrowserDate(this DateTime date)
+        {
+            var displayDate = date.ToString("o", CultureInfo.InvariantCulture);
+            return new HtmlString($"<span class='localizeDate'>{displayDate}</span>");
+        }
+        
+        public static string ToTimeAgo(this DateTimeOffset date)
+        {
+            var formatted = (DateTimeOffset.UtcNow - date).TimeString() + " ago";
+            return formatted;
+        }
+
+        public static string TimeString(this TimeSpan timeSpan)
+        {
+            if (timeSpan.TotalMinutes < 1)
+            {
+                return $"{(int)timeSpan.TotalSeconds} second{Plural((int)timeSpan.TotalSeconds)}";
+            }
+            if (timeSpan.TotalHours < 1)
+            {
+                return $"{(int)timeSpan.TotalMinutes} minute{Plural((int)timeSpan.TotalMinutes)}";
+            }
+            if (timeSpan.Days < 1)
+            {
+                return $"{(int)timeSpan.TotalHours} hour{Plural((int)timeSpan.TotalHours)}";
+            }
+            return $"{(int)timeSpan.TotalDays} day{Plural((int)timeSpan.TotalDays)}";
+        }
+
+        private static string Plural(int totalDays)
+        {
+            return totalDays > 1 ? "s" : string.Empty;
+        }
+    }
+}