[native] Handle entering edit mode when already in edit mode

Summary: We want to prevent a user from losing edited changes when entering edit mode for another message. Added displaying confirmation alert. For now, it doesn't check if there are some changes, which can be added later.

Test Plan: Turned on the app on iOS. Entered edit mode. Checked if the edit button doesn't show anymore for the currently edited message. Tried to edit another message. The confirmation alert appeared.

Reviewers: michal, inka

Reviewed By: inka

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7520