Add "is_final" attribute to learners' responses

[#72440358]
Save 'is_final' attribute of users' answers

[#72440358]
Add early support of "submitted" answers in teacher report

[#72440358]

WIP: we should show submitted stats only for required questions
(but we still don't store such information).
Add "is_required" attribute to all embeddable question types

[#72440358]
Handle "is_required" attribute of embeddables during publication

[#72440358]
Update Activity Runtime API specs, test is_require attr better

[#72440358]
Update Saveable::SaveableStandin API to include '#submitted?'

[#72440358]

So it still behaves like Saveable::Saveable.
Create new saveable answer when 'is_final' attribute is changed

[#72440358]

Previously Portal wasn't detecting any change, so it wasn't creating
a new entry in DB with submitted answer.
Add question required and answer submitted tags in teacher report

[#72440358]
Fix typo in saveable_standin_spec

[#72440358]
Make publication of external activities backwards compatible

[#72440358]

Handle 'is_required' attribute correctly when it's not provided.
Previously we were trying to set NULL when incoming hash didn't
contain it, but DB has non-null constraint on this column.