This commit removes the "Remove" and "Unsubscribe" action buttons from
various tables and replaces them with icon buttons using the "close"
icon.
Additionally, previously the rows with and without a remove button had
different heights. This commit updates the fixed `px` values to `em`
units to ensure equal height for all rows, regardless of whether a
remove-subscription button is present.
Fixes: #34874.
Instead of referring to users being added or removed from channels,
we now use subscribe or unsubscribed from channels.
Splits the article for adding and removing users from a channel
into separate articles: /help/subscribe-users-to-a-channel and
/help/unsubscribe-users-from-a-channel.
The URL redirects for the combined add/remove articles (for both
channel and stream terminology) go to the subscribe users to a
channel article.
Renames file and redirects existing links to "help/user-roles" as
the previous content tracks to that current file's content.
Updates existing links (excluding ReadtheDocs) to go to either
"help/user-roles" or "help/manage-permissions" depending on which
help center article applies to the content.
