save revision and submission history of comments

Currently, Gitcasso cannot save drafts - as soon as a tab is closed, the draft is lost.

The sketch of the proposed feature is below:

- if the user has opted-in to draft saving
- then we save the draft history of their comments in a local `IndexedDB`, with a storage model similar to Gmail
- all comments are saved forever, or you can prune to only save the past `n` months
- you can "trash" a comment, anything in trash for more than 30 days gets permanently deleted
- you can go into the trash and delete a comment immediately

There are some things you can vote on

- **I want draft-saving** -> ⭐ the repo and 👍 this issue
- **I also want the option to opt-in to draft-saving on every textarea on any website** -> 🎉 this issue
- see also #98

No files or tests are identified in the issue. Start by reviewing the existing comment textareas and related issue #98, then determine how opt-in draft saving should use local IndexedDB. Done means supporting persistent draft history, retention pruning, trash handling, and permanent deletion after 30 days.

索引モデルが issue の本文から書いたものです。