Robust previewing + update dry running for importing grade CSVs

Integrate UCSB's CSV file preview functionality - gives preview of parsed file input and allows mapping of data from file to Moodle fields. When submitted, it provides diagnosis data about:

• "potential updates", i.e. when importing a CSV will change existing grades
• errors and underlying reasons
  It should also skip duplicate scores.

Further detail - these are the strings being introduced for further detail:

• '{$a->name} not found in system mapped to {$a->key}'
• 'student {$a->name} not found in group {$a->key}'
• 'invalid grade for {$a}'
• 'missing grade for {$a}'
• 'invalid scale value for {$a}'
• 'invalid grade value for {$a}'
• 'grade exceeds item max ({$a->name}) for {$a->key}'
• 'duplicate grade for {$a->name} from {$a->key} to {$a->key2}'
• 'changing grade for {$a->name} from {$a->key} to {$a->key2}'
• 'adding grade for {$a->name} as {$a->key}'
• 'Warning: Accepted grade exceeds Item Maximum'
• 'Error: Grade exceeds Item Maximum'
• 'Error: Grade score bypassed - Locked grade'
• 'Error: Grade score bypassed - No grade'
• 'Error: Grade score bypassed - Invalid grade'
• 'Error: Grade score bypassed - Invalid group'
• 'Error: Grade score bypassed - Invalid student'
• 'Warning: Grade score bypassed - Duplicate score skipped'

Other messages

• likely used the wrong delimiter
• Non numeric grade value supplied, possibly mapped wrong column.