Fix links to IntelliJ and SBT

Fixes #1285

Author: s5bug

_includes/downloads-scala2.html

@@ -24,7 +24,7 @@ <h3>Then, install Scala:</h3>
         <img src="/resources/img/download/arrow-left.png" alt="">
         <p>Best if you prefer a full-featured IDE (recommended for beginners)</p>
       </div>
-      <a href="" class="btn-download" id="download-intellij-link">
+      <a href="https://www.jetbrains.com/idea" class="btn-download" id="download-intellij-link">
         <i class="fa fa-download"></i>
         <span>Download intellij</span>
       </a>
@@ -37,7 +37,7 @@ <h3>Then, install Scala:</h3>
         <img src="/resources/img/download/arrow-right.png" alt="">
         <p>Best if you are familiar with the command line</p>
       </div>
-      <a href="" class="btn-download" id="download-sbt-link">
+      <a href="https://www.scala-sbt.org/download.html" class="btn-download" id="download-sbt-link">
         <i class="fa fa-download"></i>
         <span>Download Sbt</span>
       </a>