Added option to checkin documents without comment.
Pep8.
Fix typo in function name.
Bump default profile version to 4000.
Correclty remove replaced checkin action.
Refactor checkin, create separate classes for each possible action.

There are four different possibilities to checkin documents:
 - checkin one document without comment
 - checkin multiple documents without comment
 - checkin one document with comment
 - checkin multiple documents with comment

 Add a sepearate view and in case of the comment form separate form classes
 for each of above actions.
Update history, move entry to correct section.
Rename CheckinCheckoutManager method `checked_out` to `get_checked_out_by`.
Merge pull request #280 from 4teamwork/lk_ogip7_opt1_checkin

Checkin comment optional.