Fulltext implementation using Bleve (#2675)

Based on #2480

This actually indexes events based on their event type. They are removed
from the index if we receive a `m.room.redaction` event on the
`OutputRoomEvent` stream.
An admin endpoint is added to reindex all existing events.


Co-authored-by: Neil Alexander <neilalexander@users.noreply.github.com>

dendrite-sample.monolith.yaml

@@ -275,10 +275,15 @@ sync_api:
   # address of the client. This is likely required if Dendrite is running behind
   # a reverse proxy server.
   # real_ip_header: X-Real-IP
-  fulltext:
+
+  # Configuration for the fulltext search
+  search:
     enabled: false
+    # The path where the fulltext index will be created in.
     index_path: "./fulltextindex"
-    language: "en" # more possible languages can be found at https://github.com/blevesearch/bleve/tree/master/analysis/lang
+    # The language most likely to be used on the server - used when indexing, to ensure the returned results match the expectations.
+    # A full list of possible languages can be found at https://github.com/blevesearch/bleve/tree/master/analysis/lang
+    language: "en"
 
 # Configuration for the User API.
 user_api: