Improve whitelist feature

* Use a primary autoincrementing primary key because primary keys need
  to be unique
* Cache whitelist on bot startup / only read from database once
* "Externalize" whitelist checking to the messageCreate function.

server/createAndLoadWhitelistTable.js

@@ -0,0 +1,48 @@
+const sqlite3 = require('sqlite3').verbose();
+module.exports = async function (clientWhitelist) { 
+    const db = new sqlite3.Database('data/database.db');
+    await new Promise ((resolve, reject)=>{
+        db.run(
+            `CREATE TABLE IF NOT EXISTS whitelist (
+                        Id INTEGER PRIMARY KEY AUTOINCREMENT, 
+                        serverId TEXT, 
+                        command TEXT,
+                        dateAdded INTEGER)`,
+            (err) => {
+                if (err) {
+                    console.error(`Error while creating table 'whitelist': ${err}`);
+                    reject(err);
+                } else {
+                    console.log("Table 'whitelist' created successfully.");
+                    resolve();
+                }
+                db.close();
+            }
+        );
+    })
+    loadWhitelist(clientWhitelist);
+}
+
+async function loadWhitelist(clientWhitelist) {
+	const db = new sqlite3.Database('data/database.db');
+	let rows = await new Promise((resolve) => {
+		db.all(`SELECT * FROM whitelist`, function (error, rows){
+			if(error){
+				console.error("Failed to read whitelist table")
+				console.error(error);
+				resolve([]);
+			}else{
+				resolve(rows);
+			}
+		});
+	});
+	rows.forEach(row => {
+		if(clientWhitelist.has(row.serverId)){
+			let oldEntry = clientWhitelist.get(row.serverId);
+			oldEntry.push(row.command)
+			clientWhitelist.set(row.serverId, oldEntry)
+		}else {
+			clientWhitelist.set(row.serverId, [row.command])
+		}
+	});
+}