Fill out basic BookmarksController

Start/Server/Extensions/ControllerExtensions.cs

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+using System;
+using System.Collections.Generic;
+using System.Security.Claims;
+using Microsoft.AspNetCore.Mvc;
+
+namespace Start.Server.Extensions {
+	public static class ControllerExtensions {
+		/// <summary>
+		/// Get the current user's ID (<see cref="ClaimTypes.NameIdentifier"/>) from claims. The
+		/// caller is assumed to have checked that the user is logged in (and thus they have a user
+		/// ID set).
+		/// <para>If there is no user ID, an exception will be thrown.</para>
+		/// </summary>
+		/// <param name="controller"></param>
+		public static string GetAuthorizedUserId(this ControllerBase controller) {
+			string? res = controller.GetUserId();
+
+			if (res == null)
+				throw new KeyNotFoundException("The user ID could not be retrieved from claims");
+
+			return res;
+		}
+
+		public static string? GetUserId(this ControllerBase controller) {
+			return controller.User.FindFirst(ClaimTypes.NameIdentifier)?.Value;
+		}
+	}
+}

Start/Server/Extensions/LinqExtensions.cs

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+using System;
+using Microsoft.EntityFrameworkCore;
+using System.Linq;
+using System.Linq.Expressions;
+
+namespace Start.Server.Extensions {
+	/// <summary>Extension methods for LINQ queries</summary>
+	public static class LinqExtensions {
+		/// <summary>
+		/// If <paramref name="condition"/> is true, then returns <c>query.Where(expression)</c>.
+		/// Otherwise returns the <paramref name="query"/> unchanged.
+		/// </summary>
+		/// <param name="query">The query to potentially apply the expression to</param>
+		/// <param name="condition">
+		/// Determines whether or not the expression should be applied
+		/// </param>
+		/// <param name="expression">A function to test each element</param>
+		public static IQueryable<T> IfWhere<T>(this IQueryable<T> query, bool condition,
+			Expression<Func<T, bool>> expression) {
+			if (condition)
+				return query.Where(expression);
+
+			return query;
+		}
+
+		/// <summary>
+		/// If the <paramref name="condition"/> is true, apply the <paramref name="transform"/> to
+		/// the query, otherwise return the query unchanged.
+		/// </summary>
+		/// <typeparam name="T">The <paramref name="query"/>'s type</typeparam>
+		/// <param name="query">The query to potentially transform</param>
+		/// <param name="condition">
+		/// If true, apply the <paramref name="transform"/> to the <paramref name="query"/>
+		/// </param>
+		/// <param name="transform">
+		/// A function to apply to the <paramref name="query"/> if the <paramref name="condition"/>
+		/// is true
+		/// </param>
+		public static IQueryable<T> If<T>(this IQueryable<T> query, bool condition,
+			Func<IQueryable<T>, IQueryable<T>> transform) {
+			if (condition)
+				return transform(query);
+
+			return query;
+		}
+	}
+}