refactor: rename canonical filter to canonicalize_url for better clarity

- Rename filter from 'canonical' to 'canonicalize_url' to follow Pongo2
naming conventions
- Update function name from filterCanonical to filterCanonicalizeURL
- Update template usage in post.html and profile.html
- Update test function name and all references

bskyweb/templates/post.html

@@ -14,7 +14,7 @@
   <meta property="profile:username" content="{{ profileView.Handle }}">
   {%- if requestURI %}
   <meta property="og:url" content="{{ requestURI }}">
-  <link rel="canonical" href="{{ requestURI|canonical }}" />
+  <link rel="canonical" href="{{ requestURI|canonicalize_url }}" />
   {% endif -%}
   {%- if postView.Author.DisplayName %}
   <meta property="og:title" content="{{ postView.Author.DisplayName }} (@{{ postView.Author.Handle }})">

bskyweb/templates/profile.html

@@ -15,7 +15,7 @@
   <meta property="profile:username" content="{{ profileView.Handle }}">
   {%- if requestURI %}
   <meta property="og:url" content="{{ requestURI }}">
-  <link rel="canonical" href="{{ requestURI|canonical }}" />
+  <link rel="canonical" href="{{ requestURI|canonicalize_url }}" />
   {% endif -%}
   {%- if profileView.DisplayName %}
   <meta property="og:title" content="{{ profileView.DisplayName }} (@{{ profileView.Handle }})">