tools (common/template): add filter to get media types:

Add 2 filters to get main type and subtype of media type. Jinja2 and Nunjucks don't handle
slices in the same way (Python way for Jinja2, JS way for Nunjucks), making it difficult
to retrieve main type of a media from media type. Thoses filters work in both cases.

libervia/backend/tools/common/template.py

@@ -482,6 +482,8 @@ class Renderer(object):
         self.env.filters["highlight"] = self.highlight
         self.env.filters["front_url"] = (self._front_url if front_url_filter is None
                                          else front_url_filter)
+        self.env.filters["media_type_main"] = self.media_type_main
+        self.env.filters["media_type_sub"] = self.media_type_sub
         # custom tests
         self.env.tests["in_the_past"] = self._in_the_past
         self.icons_path = os.path.join(host.media_dir, "fonts/fontello/svg")
@@ -924,6 +926,18 @@ class Renderer(object):
         formatter = formatters.HtmlFormatter(**html_fmt_opts)
         return safe(pygments.highlight(code, lexer, formatter))
 
+    def media_type_main(self, value: Optional[str]) -> Optional[str]:
+        """Return main type of a media type"""
+        if not value:
+            return None
+        return value.partition("/")[0]
+
+    def media_type_sub(self, value: Optional[str]) -> Optional[str]:
+        """Return main type of a media type"""
+        if not value:
+            return None
+        return value.partition("/")[1]
+
     ## custom tests ##
 
     def _in_the_past(self, timestamp):