htdocs: Update selectize to 0.15.2

a741c95 upgraded selectize to 0.13.5. However, it looks like a
non-standalone version was uploaded by accident. On page load, the
following error can be seen in debug console: "Selectize: Dependency
MicroPlugin is missing".

Because of that, the submitter autocomplete no longer works.

Version 0.13.5 was not released on Github, so I have decided to upgrade
the library to the latest available version.

Add updated files to .gitattributes to skip them in patches.

Author: fstachura

.gitattributes

@@ -0,0 +1,2 @@
+htdocs/js/*.min.js binary
+htdocs/css/selectize.bootstrap3.css binary

htdocs/README.rst

@@ -140,4 +140,4 @@ js
 
   :Website: https://selectize.github.io/selectize.js/
   :GitHub: https://github.com/selectize/selectize.js
-  :Version: 0.13.5
+  :Version: 0.15.2