[MM-22314] Remove read limit at app/import which cause bulk import to fail (#13863)

* app/import: increase buffer size to avoid failing in large inputs

* remove commented out lock

* app/import: make maxTokenSize constant

Co-authored-by: mattermod <mattermod@users.noreply.github.com>

app/import_test.go

@@ -190,6 +190,19 @@ func TestImportBulkImport(t *testing.T) {
 	require.NotNil(t, err, "Should have failed due to missing version line on line 1.")
 	require.Equal(t, 1, line, "Should have failed due to missing version line on line 1.")
 
+	// Run bulk import using a valid and large input and a \r\n line break.
+	t.Run("", func(t *testing.T) {
+		posts := `{"type": "post"` + strings.Repeat(`, "post": {"team": "`+teamName+`", "channel": "`+channelName+`", "user": "`+username+`", "message": "Repeat after me", "create_at": 193456789012}`, 1E4) + "}"
+		data4 := `{"type": "version", "version": 1}
+{"type": "team", "team": {"type": "O", "display_name": "lskmw2d7a5ao7ppwqh5ljchvr4", "name": "` + teamName + `"}}
+{"type": "channel", "channel": {"type": "O", "display_name": "xr6m6udffngark2uekvr3hoeny", "team": "` + teamName + `", "name": "` + channelName + `"}}
+{"type": "user", "user": {"username": "` + username + `", "email": "` + username + `@example.com", "teams": [{"name": "` + teamName + `","theme": "` + teamTheme1 + `", "channels": [{"name": "` + channelName + `"}]}]}}
+{"type": "post", "post": {"team": "` + teamName + `", "channel": "` + channelName + `", "user": "` + username + `", "message": "Hello World", "create_at": 123456789012}}`
+		err, line = th.App.BulkImport(strings.NewReader(data4+"\r\n"+posts), false, 2)
+		require.Nil(t, err, "BulkImport should have succeeded")
+		require.Equal(t, 0, line, "BulkImport line should be 0")
+	})
+
 	t.Run("First item after version without type", func(t *testing.T) {
 		data := `{"type": "version", "version": 1}
 {"name": "custom-emoji-troll", "image": "bulkdata/emoji/trollolol.png"}`