add build time as a query parameter for scripts.js and styles.css

This forces browsers to fetch the files again when the package has been
rebuilt.

bin/numbas.py

@@ -106,6 +106,8 @@ def get_theme_path(self, theme):
     def compile(self):
         self.parse_exam()
 
+        self.build_time = datetime.datetime.now()
+
         files = self.files = self.collect_files()
 
         self.render_templates()
@@ -288,7 +290,7 @@ def render_templates(self):
     def render_template(self, name):
         try:
             template = self.template_environment.get_template(name)
-            output = template.render({'exam': self.exam, 'options': self.options})
+            output = template.render({'exam': self.exam, 'options': self.options, 'build_time': self.build_time.timestamp()})
             return output
         except jinja2.exceptions.TemplateNotFound:
             return None
@@ -441,7 +443,7 @@ def compileToZip(self):
             dst = ZipInfo(str(Path(dst).relative_to('.')))
             dst.compress_type = zipfile.ZIP_DEFLATED
             dst.external_attr = 0o644<<16
-            dst.date_time = datetime.datetime.today().timetuple()
+            dst.date_time = self.build_time.timetuple()
             if isinstance(src, Path):
                 f.writestr(dst, src.read_bytes())
             else:

themes/default/templates/scripts.html

@@ -21,7 +21,7 @@
 <script type="text/javascript" src="{{options.mathjax_url}}/MathJax.js?config=TeX-AMS-MML_HTMLorMML.js"></script>
 
 <!-- numbas stuff -->
-<script charset="UTF-8" type="text/javascript" src="scripts.js" charset="utf-8"></script>
+<script charset="UTF-8" type="text/javascript" src="scripts.js?build_time={{build_time}}" charset="utf-8"></script>
 {% if not dont_start_exam %}
 <script>
     Numbas.queueScript('go',['start-exam'],function() {

themes/default/templates/stylesheets.html

@@ -6,4 +6,4 @@
         display: none;
     }
 </style>
-<link rel="stylesheet" type="text/css" href="styles.css" />
+<link rel="stylesheet" type="text/css" href="styles.css?build_time={{build_time}}" />