This commit adds support to display `realm_empty_topic_display_name`
value in the confirm_merge_topics_with_rename modal for topics having
the actual value of empty string.
Updates translated strings in web/ that do not need updates to any
tests. The majority of these strings are also unique to the file/
template that they are in. A few have overlap with one other file.
Some changes here update placeholders/variables in these strings to
no longer use stream so that all the translation updates for this
rename happen at the same time.
The exception to this are cases of "<z-stream>" placeholders in
these translated strings.
Part of the stream to channel rename project.
Fixes#27369.
When editing a topic name via the topic header bar, it's easy to
accidentally move messages into an existing topic, which is
difficult to undo. This commit adds a confirmation modal for when
this is about to happen.
