Fix hashtag search results (#576)

We mistakenly wrote to the key "hashtag" instead of "hashtags", resulting
in no results in the API response. Additionally, the type of the Tag's `history`
needs to be a list, not a dict.

This fixes hashtag search in Elk.

api/schemas.py

@@ -275,7 +275,7 @@ class Notification(Schema):
 class Tag(Schema):
     name: str
     url: str
-    history: dict
+    history: list
     following: bool | None
 
     @classmethod

api/views/search.py

@@ -39,7 +39,7 @@ def search(
             for i in search_result["identities"]
         ]
     if type is None or type == "hashtag":
-        result["hashtag"] = [
+        result["hashtags"] = [
             schemas.Tag.from_hashtag(h) for h in search_result["hashtags"]
         ]
     if type is None or type == "statuses":