Wrap outbound HTML mention links in span

This is Mastodon specific, but what can we do than copy the
biggest for greater compatibility.

federation/entities/mixins.py

@@ -218,7 +218,7 @@ class RawContentMixin(BaseEntity):
                         display_name = mention
                     rendered = rendered.replace(
                         "@{%s}" % mention,
-                        f'@<a href="{mention}" class="mention">{display_name}</a>',
+                        f'@<a href="{mention}" class="mention"><span>{display_name}</span></a>',
                     )
             # Finally linkify remaining URL's that are not links
             rendered = process_text_links(rendered)

federation/tests/entities/activitypub/test_entities.py

@@ -181,7 +181,7 @@ class TestEntitiesConvertToAS2:
                 'attributedTo': 'http://127.0.0.1:8000/profile/123456/',
                 'content': '<h1>raw_content</h1>\n<p>@{someone@localhost.local} @<a class="mention" '
                            'href="http://localhost.local/someone" rel="nofollow" target="_blank">'
-                           'Bob Bobértson</a></p>',
+                           '<span>Bob Bobértson</span></a></p>',
                 'published': '2019-04-27T00:00:00',
                 'inReplyTo': None,
                 'sensitive': False,