Add a management command to tag settlement schools

[higs4281]

We need a way to tag new groups of settlement schools as they come in,
and this provides a way to do it via CSV, without having to deploy code.
It also affords a way to untag schools that had been marked as
settlement schools for demo purposes. Schools in this state have a
settlement_school value of demo, and by setting up a CSV with all the
demo schools and providing a blank tag value, we can de-tag all the demo
schools in one go.

Additions

• Script, associated management command and tests

Testing

• Try tagging some test schools locally with

./manage.py tag_schools "http://files.consumerfinance.gov.s3.amazonaws.com/pb/paying_for_college/csv/tag_test.csv"

You should get a message saying two schools were tagged.

Review

• @amymok @willbarton
• Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)

[coveralls]

Coverage remained the same at 100.0% when pulling aa3a66e on tag-schools into b9fbbbf on master.

[amymok]

👍

[coveralls]

Coverage remained the same at 100.0% when pulling 2d5b3e6 on tag-schools into f2ec9a8 on master.

[coveralls]

Coverage remained the same at 100.0% when pulling a508e49 on tag-schools into f2ec9a8 on master.

[coveralls]

Coverage remained the same at 100.0% when pulling 0c2aec3 on tag-schools into f2ec9a8 on master.

[coveralls]

Coverage remained the same at 100.0% when pulling 416eab9 on tag-schools into f2ec9a8 on master.

[coveralls]

Coverage remained the same at 100.0% when pulling 0e85b45 on tag-schools into f2ec9a8 on master.

[coveralls]

Coverage remained the same at 100.0% when pulling 3c68be3 on tag-schools into f2ec9a8 on master.

[coveralls]

Coverage remained the same at 100.0% when pulling 8d93ad4 on tag-schools into f2ec9a8 on master.