Fix errors found by htmlproofer

It's mostly missing alt attributes on images and
some errors on reveal.js notes server component which
I am not actually using.

presentations/agile_and_friends/index.html

@@ -142,7 +142,7 @@ <h3>The birth of the <i>Agile Manifesto</i></h3>
 
         <section>
           <h2>The Agile Manifesto</h2>
-          <a href="http://agilemanifesto.org/">agilemanifesto.org</a>
+          <a href="https://agilemanifesto.org/">agilemanifesto.org</a>
         </section>
 
         <section>
@@ -232,7 +232,7 @@ <h2>The Agile Manifesto</h2>
           <h2>Scrum</h2>
           <p class="fragment" data-fragment-index="0">
             Sprint - timeboxed effort
-            <img src='sprints.png' style="border: none; box-shadow: none"/>
+            <img src='sprints.png' alt="Image of a sprint on a timeline" style="border: none; box-shadow: none" />
           </p>
           <div class="fragment scrum" data-fragment-index="1" style="width: 50%">
             Sprint planning

presentations/business_logic/index.html

@@ -128,7 +128,7 @@ <h2>Business logic is usually the fastest changing code in the system</h2>
         </section>
 
         <section>
-          <img src="./images/MVC-Process.svg" />
+          <img src="./images/MVC-Process.svg" alt="MVC process" />
         </section>
 
         <section>
@@ -155,7 +155,7 @@ <h3>All in the models</h3>
           </section>
 
           <section>
-            <img src="./images/all-in-the-model.svg" />
+            <img src="./images/all-in-the-model.svg" alt="All logic in the model" />
           </section>
 
           <section>
@@ -189,7 +189,7 @@ <h3>All in the controllers</h3>
           </section>
 
           <section>
-            <img src="./images/all-in-controllers.svg" />
+            <img src="./images/all-in-controllers.svg" alt="All logic in the controller" />
           </section>
 
           <section>
@@ -220,7 +220,7 @@ <h3>Service objects</h3>
           </section>
 
           <section>
-            <img src="./images/service-objects.svg" />
+            <img src="./images/service-objects.svg" alt="Service objects" />
           </section>
 
           <section>
@@ -252,7 +252,7 @@ <h3>Form / Business action objects</h3>
           </section>
 
           <section>
-            <img src="./images/ba-objects.svg" />
+            <img src="./images/ba-objects.svg" alt="Business action objects" />
           </section>
 
           <section>