We introduce a different class to be used for left sidebar which
inherits TopicListWidget. More specifically, we allow the
children classes of TopicListWidget to pass their own method to
filter topics. This will later be used by inbox view when showing
channel view in center pane.
We show `realm_empty_topic_display_name` for empty string topics
in the left sidebar.
This commit makes it possible for users to search for
the `realm_empty_topic_display_name` value to filter out empty
string topics.
This commit updates the topic links obtained from clicking
the topics in the left sidebar, recent view and inbox, and
those obtained from "Copy link to topic" to use the new
topic permalinks.
Fixes part of #21505.
