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>

internal/fulltext/bleve_test.go

@@ -27,11 +27,11 @@ import (
 
 func mustOpenIndex(t *testing.T, tempDir string) *fulltext.Search {
 	t.Helper()
-	cfg := config.Fulltext{}
-	cfg.Defaults(config.DefaultOpts{
-		Generate:   true,
-		Monolithic: true,
-	})
+	cfg := config.Fulltext{
+		Enabled:  true,
+		InMemory: true,
+		Language: "en",
+	}
 	if tempDir != "" {
 		cfg.IndexPath = config.Path(tempDir)
 		cfg.InMemory = false