message_edit: Separate permission checks for topic edits.
message_edit: Separate permission checks for topic edits.

We separate the permission checks for content and topic edits
by changing the can_edit_topic_or_content to can_edit_topic
and use it only for checking topic edits and check content
edits separately in check_update_message itself. There is no
change in behavior as of this PR, there will be more changes
as per #21739.

This is a prep commit for #21739. The permission checks for
them are essentially separate except the one that message
sender is allowed to edit content and topic irresepctive of
edit_topic_policy setting, and this will too be changed in
future commit and so it will be better to have these checks
separate for readability.

We can also probably create a new function for checking content
edits but currently we only check the sender is same as the use
who is editing and it does not make sense to have a separate
function for just one check. We can do so in future in case we
do some more refactoring for #21739.

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