Use calc's evaluator for staff answers in `Numercial` type

Rather than just calling `complex(...)` on the staff answers, also call
`calc.evaluator`. This way, simple things like `1/3` can be entered, as well
as more complicated explicit equations (e.g. `sqrt((12^2 + 26^2)/2)` etc, etc)

Simplify and update the tests to work with this.
Incorporate Anant's suggestions
Persist static_tab name.

STUD-663.
Add doc strings and fix spaces
Merge pull request #741 from edx/chirstina/static-tab-rc

Persist static_tab name.
indent even more
Make it so that students who are not enrolled cannot see the forum
Let staff have access to a forum even if they're not enrolled in the course.
address some pylint violations in the mixedmodulestore build report
Merge pull request #744 from edx/dhm/pep_pylint_cleanups

Add doc strings and fix spaces
Merge pull request #735 from edx/fix/cdodge/pylint-violations

address some pylint violations in the mixedmodulestore build report
adds in the .reset type of action to the x module capa styling
Merge pull request #748 from edx/talbs/lms-fix-resetbutton

Sync up Reset Action Button Styling for Capa xmodules
Merge pull request #726 from edx/peterb/numerical/evaluate-staff

Use calc's evaluator for staff answers in `Numercial` type
Merge pull request #731 from edx/jkarni/feature/docchanges

Incorporate Anant's suggestions
Fix failing grade label being editable but not saved.
Add acceptance test for noneditable failing grade range.
Add docs for changes in #726
Merge branch 'release'
Merge pull request #749 from edx/peter-fogg/fix-rename-fail-grade

Fix failing grade label being editable but not saved.