Resolve character encoding issues in StudentsImporter and BehaviorImporter

[kevinrobinson]

Examples:

somerville:BehaviorImporter
  failed to convert 143 unexpected invalid character encodings for ["\x93", "\x94", "\xF3", "\xED", "\xE1", "\xFA", "\xC1", "\x95", "\xA8", "\xF1", "\x97", "\x96", "\x85", "\xA0"]

new-bedford:BehaviorImporter
failed to convert 9389 unexpected invalid character encodings for ["\xE2", "\x80", "\x99", "\xA6", "\x9C", "\x9D", "\x98", "\x93", "\xC2", "\xA0", "\xA2", "\xC3", "\xA9", "\xB3", "\xA3", "\xBD"]

somerville:StudentsImporter
  failed to convert 2 unexpected invalid character encodings for ["\xF8"]