[HTML] Add download button (#235)

This adds a download button to the HTML version of the book, which enables
downloading the html, pdf and epub version of the book.

Makefile

@@ -18,11 +18,13 @@ COMMONFILTERS = \
           --citeproc
 
 .PHONY: all clean pdf html
-all: pdf html default_pandoc_html_template default_pandoc_latex_template native epub
+all: pdf html default_pandoc_html_template default_pandoc_latex_template native epub \
+     downloads
 pdf: build/book.pdf
 html: build/book.html build/default.css build/index.html
 native: build/book.native
 epub: build/book.epub
+downloads: build/LLSoftSecBook.pdf build/LLSoftSecBook.html build/LLSoftSecBook.epub
 
 # The source of images are in SVG format.
 # The below lines define to convert the SVG source images to PDF images such
@@ -69,6 +71,15 @@ build/book.epub: book.md book.bib Makefile build
 build/index.html: build/book.html build
 	cp build/book.html build/index.html
 
+build/LLSoftSecBook.pdf: build/book.pdf build
+	cp build/book.pdf build/LLSoftSecBook.pdf
+
+build/LLSoftSecBook.html: build/book.html build
+	cp build/book.html build/LLSoftSecBook.html
+
+build/LLSoftSecBook.epub: build/book.epub build
+	cp build/book.epub build/LLSoftSecBook.epub
+
 build/book.tex: book.md book.bib Makefile build theme/tex/pandoc_template.tex \
 				theme/html/markup_issue.lua \
 				theme/html/markup_todo.lua \

theme/html/pandoc_template.html

@@ -63,6 +63,20 @@
       data-bs-placement="bottom">
       <span class="navbar-toggler-icon"></span>
     </a>
+    <span class="dropdown" id="download-dropdown">
+      <button
+        class="btn btn-outline-primary btn-sm dropdown-toggle"
+        id="download-button"
+        type="button" id="downloadDropdownMenuButton"
+        data-bs-toggle="dropdown" aria-haspopup="true" aria-expanded="false"
+        >
+        <i class="bi bi-download"></i></button>
+      <div class="dropdown-menu" aria-labelledby="downloadDropdownMenuButton">
+        <a id="html-download-link" class="dropdown-item" href="LLSoftSecBook.html" target="_blank">HTML</a>
+        <a id="pdf-download-link" class="dropdown-item" href="LLSoftSecBook.pdf" target="_blank">PDF</a>
+        <a id="epub-download-link" class="dropdown-item" href="LLSoftSecBook.epub" target="_blank">EPub</a>
+      </div>
+    </span>
     <a id="view-source-button"
       class="btn btn-sm btn-outline-primary" role="button"
       data-bs-placement="bottom"