message_edit: Fix message edit history breakdown on large messages.
message_edit: Fix message edit history breakdown on large messages.

The message_edit_history UI was incorrectly inheriting its content
div's structure from message_edit_content, i.e. the form for editing
message content, and not message_content, i.e. the class for rendering
the content of messages in the message feed (which we also use for
drafts).

Fix this by changing the inheritance, while also adding a (currently
unused) class for any future customizations.

Fixes: #5629.

eeshangarg/zulip,synicalsyntax/zulip,rishig/zulip,rht/zulip,kou/zulip,kou/zulip,hackerkid/zulip,rishig/zulip,rht/zulip,kou/zulip,hackerkid/zulip,kou/zulip,andersk/zulip,brainwane/zulip,synicalsyntax/zulip,shubhamdhama/zulip,rishig/zulip,rishig/zulip,tommyip/zulip,andersk/zulip,kou/zulip,brainwane/zulip,hackerkid/zulip,zulip/zulip,eeshangarg/zulip,punchagan/zulip,rht/zulip,zulip/zulip,shubhamdhama/zulip,eeshangarg/zulip,brainwane/zulip,hackerkid/zulip,punchagan/zulip,andersk/zulip,timabbott/zulip,eeshangarg/zulip,synicalsyntax/zulip,brainwane/zulip,tommyip/zulip,shubhamdhama/zulip,brainwane/zulip,eeshangarg/zulip,punchagan/zulip,synicalsyntax/zulip,brainwane/zulip,rishig/zulip,tommyip/zulip,andersk/zulip,rht/zulip,punchagan/zulip,tommyip/zulip,hackerkid/zulip,timabbott/zulip,rishig/zulip,tommyip/zulip,eeshangarg/zulip,hackerkid/zulip,punchagan/zulip,synicalsyntax/zulip,rht/zulip,showell/zulip,showell/zulip,kou/zulip,showell/zulip,andersk/zulip,kou/zulip,andersk/zulip,tommyip/zulip,timabbott/zulip,shubhamdhama/zulip,rht/zulip,zulip/zulip,showell/zulip,brainwane/zulip,andersk/zulip,zulip/zulip,rht/zulip,showell/zulip,eeshangarg/zulip,zulip/zulip,punchagan/zulip,hackerkid/zulip,rishig/zulip,synicalsyntax/zulip,shubhamdhama/zulip,zulip/zulip,timabbott/zulip,timabbott/zulip,showell/zulip,zulip/zulip,tommyip/zulip,punchagan/zulip,timabbott/zulip,showell/zulip,shubhamdhama/zulip,timabbott/zulip,shubhamdhama/zulip,synicalsyntax/zulip