Support for showanswer settings

[pdpinch]

As a course author, I would like to be able to use showanswer settings with SGA. In particular, I would like to use the Attempted setting so that the learner is presented with a solution to the problem after choosing to submit it.

Acceptance criteria

• [ ] add showanswer field to SGA XBlock fields
• [ ] for backwards compatibility, if there is no showanswer value it should default to "Past Due"
• [ ] add solution field to SGA XBlock. This will contain the solution.
• [ ] implement OLX serialization, so that the showanswer and solution settings can be exported and imported
• [ ] #186 Implement a submit button, distinct from the upload button, so the learner can upload a file, review it and then choose to submit. After submission, the file cannot be changed

Out of scope

• Not all the supported values for showanswer make sense in the SGA context. We'll only implement past_due and attempted

[pdpinch]

One approach to doing this would be to:

1. Fork xblock-utils. This xblock library is helpful because it can provide the studio interface for our changes.
2. Copy & paste the relevant code from edx-platform into xblock-utils
3. import our forked xblock-utils into edx-sga

edX asked that we submit our changes to xblock-utils upstream and also update capa_base to use them. However, this is out of scope for this issue.

[pdpinch]

Here is the relevant code from capa_base.py:

1. constants: https://github.com/edx/edx-platform/blob/master/common/lib/xmodule/xmodule/capa_base.py#L29
2. UI https://github.com/edx/edx-platform/blob/master/common/lib/xmodule/xmodule/capa_base.py#L133-L148
3. answer_available() https://github.com/edx/edx-platform/blob/master/common/lib/xmodule/xmodule/capa_base.py#L892-L925
4. get_answer() https://github.com/edx/edx-platform/blob/master/common/lib/xmodule/xmodule/capa_base.py#L996-L1037
5. get_problem_html() -- template rendering https://github.com/edx/edx-platform/blob/master/common/lib/xmodule/xmodule/capa_base.py#L733