font color → span mx color for outgoing HTML

Also remove HTML_PROCESSOR `filter_inline` and `from_markdown_inline`
methods. `filter` and `from_markdown` now take an `inline` argument.

src/backend/models/items.py

@@ -239,7 +239,8 @@ class Event(ModelItem):
 
     def __post_init__(self) -> None:
         if not self.inline_content:
-            self.inline_content = HTML_PROCESSOR.filter_inline(self.content)
+            self.inline_content = \
+                HTML_PROCESSOR.filter(self.content, inline=True)
 
 
     def __lt__(self, other: "Event") -> bool: