widget: Use different variable names for message and submessage content.
widget: Use different variable names for message and submessage content.

This commit renames the content variable in do_widget_post_save_actions
to message_content and is a prep commit for changing the return type of
check_message from Dict to dataclass.

This change is required because content variable is used two times in
this function - one for message content and other for submessage
content, so when we change the return type of check_message to
dataclass, the type of content variable is considered as str and then
when dict is assigned to content in the submessage case, mypy raises
'Incompatible types in assignment' error.

This issue is not faced before the dataclass migration because there is
no type checking for the values of dict returned by check_message as the
return type of check_message is 'Dict[str, Any]'.

kou/zulip,andersk/zulip,eeshangarg/zulip,punchagan/zulip,punchagan/zulip,rht/zulip,eeshangarg/zulip,rht/zulip,eeshangarg/zulip,hackerkid/zulip,punchagan/zulip,zulip/zulip,andersk/zulip,hackerkid/zulip,zulip/zulip,andersk/zulip,punchagan/zulip,punchagan/zulip,rht/zulip,punchagan/zulip,kou/zulip,zulip/zulip,eeshangarg/zulip,rht/zulip,hackerkid/zulip,rht/zulip,zulip/zulip,zulip/zulip,hackerkid/zulip,punchagan/zulip,hackerkid/zulip,rht/zulip,andersk/zulip,andersk/zulip,kou/zulip,andersk/zulip,kou/zulip,kou/zulip,hackerkid/zulip,hackerkid/zulip,zulip/zulip,kou/zulip,kou/zulip,eeshangarg/zulip,andersk/zulip,eeshangarg/zulip,eeshangarg/zulip,rht/zulip,zulip/zulip