Enable finding existing hashtags case-insensitive

We should store hashtags case-sensitive, but ensures that an existing
hashtag with different case are found and re-used. for example,
an existing #BookWyrm hashtag will be found and used even if the
status content is using #bookwyrm.

bookwyrm/views/status.py

@@ -116,7 +116,7 @@ class CreateStatus(View):
             status.mention_users.add(status.reply_parent.user)
 
         # inspect the text for hashtags
-        for (mention_text, mention_hashtag) in find_hashtags(content).items():
+        for (mention_text, mention_hashtag) in find_or_create_hashtags(content).items():
             # add them to status mentions fk
             status.mention_hashtags.add(mention_hashtag)
 
@@ -250,25 +250,36 @@ def find_mentions(user, content):
     return username_dict
 
 
-def find_hashtags(content):
-    """detect #hashtags in raw status content"""
+def find_or_create_hashtags(content):
+    """detect #hashtags in raw status content
+
+    it stores hashtags case-sensitive, but ensures that an existing
+    hashtag with different case are found and re-used. for example,
+    an existing #BookWyrm hashtag will be found and used even if the
+    status content is using #bookwyrm.
+    """
     if not content:
         return {}
 
-    hashtags = re.findall(regex.HASHTAG, content)
-    if len(hashtags) == 0:
+    found_hashtags = {t.lower(): t for t in re.findall(regex.HASHTAG, content)}
+    if len(found_hashtags) == 0:
         return {}
 
-    known_tags = models.Hashtag.objects.filter(Q(name__in=hashtags)).distinct()
-    hashtag_dict = {t.name: t for t in known_tags}
+    known_hashtags = {
+        t.name.lower(): t
+        for t in models.Hashtag.objects.filter(
+            Q(name__in=found_hashtags.keys())
+        ).distinct()
+    }
 
-    not_found = set(hashtags) - set(hashtag_dict.keys())
-    for tag_name in not_found:
+    not_found = found_hashtags.keys() - known_hashtags.keys()
+    for lower_name in not_found:
+        tag_name = found_hashtags[lower_name]
         mention_hashtag = models.Hashtag(name=tag_name)
         mention_hashtag.save()
-        hashtag_dict[mention_hashtag.name] = mention_hashtag
+        known_hashtags[lower_name] = mention_hashtag
 
-    return hashtag_dict
+    return {found_hashtags[k]: v for k, v in known_hashtags.items()}
 
 
 def format_links(content):