AssessmentImporter: Update MCAS for Bedford

[kevinrobinson]

Who is this PR for?

Bedford educators

What problem does this PR fix?

Folks in Bedford want to import MCAS assessment data, but the format of their export is different than Somerville's, so we can't just turn this on.

What does this PR do?

Makes the MCAS import process work differently using PerDistrict:

1. Update the X2AssessmentImporter to parse different things within the same file with PerDistrict#choose_assessment_importer_row_class.
2. Update the McasRow class to be more explicit about matching, factors out differences across districts into PerDistrict#normalized_subject_from_mcas_export.

Some other related cleanup:
3. Removes sanitization of assessment_growth currently applied across all assessments, since it conflicts with Bedford MCAS data. For now this is just removed so the only guard is database type constraint (there are no validations).
4. Removes :assessment_date as a field handled by CsvRowCleaner, and push this out to individual importers (eg, DibelsRow and AccessRow). This is aiming to remove that class altogether to make importers more explicit about any transformations.

Checklists

Which features or pages does this PR touch?

• Profile
• Overview
• X2AssessmentImporter

Does this PR use tests to help verify we can deploy these changes quickly and confidently?

• Included specs for changes
• Improved specs for existing code in need of better test coverage

[kevinrobinson]

hmm, got what looks like a flaky test error: https://travis-ci.org/studentinsights/studentinsights/builds/448425967?utm_source=github_status&utm_medium=notification, going to restart it for now and ignore

[kevinrobinson]

selfie