Store diff with line note. It makes possible to see inline notes with proper diff even if MR diff changed

Most of these are comments but a few are strings for users.

Might be an idea to run this from time to time:
https://github.com/lyda/misspell-check

It runs mostly clean now.

* remove length restriction on note
* add format validation for line_code

Without lambdas, Date.today will be evaluated in the class body.
For it to have a running scope of last week etc, it will need to
be evaluated each time the scope is called.
In Rails 4.0, lambdas will be required for all scopes, so not a bad
idea to go ahead and change them all now.