Redesign solution approval mechanism

[tcibinan]

Summary

Solution approvals was introduced in #4.

It turned out that the original implementation does not suit tutor needs very well.

First of all, a solution can only be in one of two states: approved or disapproved. Therefore, as long as solution has been created it instantly becomes disapproved. It may be confusing and even adverse. F.e. if tutor wants to request changes on a reviewed newest solution he has to mark a solution as approved to be able to mark it as disapproved after.

Moreover, it brings some inconsistency with pull request reviews. Once created a solution has a disapproved status in flaxo but does not have any backing pull request review after it.

The last problem lies in solutions approval UI representation. Only checkboxes are displayed bringing no addition information about the message or the review creation date.

Proposal

Solution approval should have three possible state: none, dissaproved and approved. The last two states can have an associated message and it should be visible for a tutor as well as editable while approval creation.

The idea is to replace a single instanced solution approval with a series of approvals. Each one of the approvals should have a backing pull request review. The series can be shown in the task solutions table after a clicking on a specific solution row. An approval creation button should be right at the end of the approval series.