Research modify commit message behavior

This was discussed on slack, going to list the ideas:

Problem

Lack of feedback when modifying commit message in merge request.

When the user clicks "Modify commit message", a textarea is presented, but there is no save button. The user can't change the message and navigate to other page and then comeback to merge the MR without losing the message.

• Currently we don't have a way to save the message changed.
• It's a little confusing because user has no feedback if the message changed was saved/will be used.
• If the user could save the message, it could navigate to other pages and get back to merge without losing the message.

Results

From the discussion on slack these are the results:

• save it in local storage, like we do with issue descriptions and comments
• it might be confusing that the button is changing the merge commit message, we could try to make it more clear:
  • change the button title to Modify merge commit message
  • "Modify" anywhere (vs "Edit") - "Modify merge commit message" has too much alliteration for me

Might be worth adding it into the research panel.

cc @sarrahvesselov @sarahod