Track trending tags (#7638)

* Track trending tags

- Half-life of 1 day
- Historical usage in daily buckets (last 7 days stored)
- GET /api/v1/trends

Fix #271

* Add trends to web UI

* Don't render compose form on search route, adjust search results header

* Disqualify tag from trends if it's in disallowed hashtags setting

* Count distinct accounts using tag, ignore silenced accounts

app/javascript/mastodon/reducers/trends.js

@@ -0,0 +1,13 @@
+import { TRENDS_FETCH_SUCCESS } from '../actions/trends';
+import { fromJS } from 'immutable';
+
+const initialState = null;
+
+export default function trendsReducer(state = initialState, action) {
+  switch(action.type) {
+  case TRENDS_FETCH_SUCCESS:
+    return fromJS(action.trends);
+  default:
+    return state;
+  }
+};