Have Proposals editable before acceptance

[dternyak]

In ZF Grants 2.1, Proposals may have changes requested by administrators while it is in an "Open for Public Review" state. This means that proposers won't be able to edit their proposal as if it is in a draft state during until an administrator requests changes.

It would be useful to allow proposers to modify their proposal on an ongoing basis while it is in an "Open for Public Review State". For transparency and auditability, a "Revision History" functionality should be added to keep track of changes that have been made to the proposal once it has been made public.

In short: Admins will continue to be able to request changes to a proposal to encourage proposers to modify their proposals in specific ways, but the proposers will be able to make edits without admin request as well.

(Based on feedback from @tromer)