Added .imageonlychannel / .imageonly to prevent users from posting anything but images in the channel

src/NadekoBot/Migrations/20210914180026_image-only-channels.cs

@@ -0,0 +1,38 @@
+using System;
+using Microsoft.EntityFrameworkCore.Migrations;
+
+namespace NadekoBot.Migrations
+{
+    public partial class imageonlychannels : Migration
+    {
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.CreateTable(
+                name: "ImageOnlyChannels",
+                columns: table => new
+                {
+                    Id = table.Column<int>(type: "INTEGER", nullable: false)
+                        .Annotation("Sqlite:Autoincrement", true),
+                    GuildId = table.Column<ulong>(type: "INTEGER", nullable: false),
+                    ChannelId = table.Column<ulong>(type: "INTEGER", nullable: false),
+                    DateAdded = table.Column<DateTime>(type: "TEXT", nullable: true)
+                },
+                constraints: table =>
+                {
+                    table.PrimaryKey("PK_ImageOnlyChannels", x => x.Id);
+                });
+
+            migrationBuilder.CreateIndex(
+                name: "IX_ImageOnlyChannels_ChannelId",
+                table: "ImageOnlyChannels",
+                column: "ChannelId",
+                unique: true);
+        }
+
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropTable(
+                name: "ImageOnlyChannels");
+        }
+    }
+}