footer: Split into fixed elements

Instead of having the link to the homepage (via the logo) and the theme
switcher in the footer, split this up. The logo is now in the top left
and the theme toggle in the bottom right. The footer itself is removed
on the homepage, but kept for blog posts, guides, etc.

_includes/fixed.html

@@ -0,0 +1,4 @@
+<a href="/">
+  <img id="logo" src="/assets/images/radicle.svg" width="32" height="32" />
+</a>
+<button class="toggle" id="toggle-theme"><!-- Set by script --></button>

_includes/footer.html

@@ -1,7 +0,0 @@
-<footer>
-  <a href="/">
-    <img id="logo" src="/assets/images/radicle.svg" width="32" height="32" style="margin-top: 4px"/>
-  </a>
-  <span><small>© <span class="desktop">2018–{{ 'now' | date: "%Y" }}</span> The Radicle Team</small></span>
-  <span><button class="toggle" id="toggle-theme"><!-- Set by script --></button></span>
-</footer>