[mimecast] Cleanup field mappings and package description

[andrewkroh]

What does this PR do?

• Readme:
  • Format and sort readme sections
• Dashboard:
  • Update dashboard field name
  • And fix a typo in mime_type.
• Make integration description consistent
• ttp_url_logs cleanup
  • Use ECS fields for ttp_url_logs.
  • Add missing event.created mapping.
• ttp_ip_logs cleanup
  • Use ECS field definitions for ttp_ip_logs.
  • Add missing event.created mapping.
• ttp_ap_logs cleanup
  • Use ECS field definitions for ttp_ap_logs.
  • It was using email.attachments.hash which is not a valid ECS field so I changed it to use email.attachments.file.hash.sha256.
• threat_intel_malware_grid cleanup
  • Add missing ECS event field mappings.
• threat_intel_malware_customer cleanup
  • Add missing ECS event fields.
• siem_logs cleanup
  • Use ECS fields in siem_logs.
  • Remove email.message_size and email.header_from which are not part of ECS. Use mimecast.MsgSize and email.from.address instead.
  • Add allow_duplicates: false to email.{to,from}.address append processors.
  • Remove source.as.asn and source.as.organization_name and use the correct ECS fields.
• dlp_logs cleanup
  • Use ECS fields for dlp_logs.
  • None of the mimecast.* fields are used so remove the mappings.
• audit_logs cleanup
  • Use ECS fields for audit_logs.
  • Remove unused source.as.{asn,organization_name}.

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• I have verified that all data streams collect metrics or logs.
• I have added an entry to my package's changelog.yml file.
• I have verified that Kibana version constraints are current according to guidelines.

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Pinging @elastic/security-external-integrations (Team:Security-External Integrations)

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• Start Time: 2022-04-26T12:44:56.702+0000

• Duration: 17 min 3 sec

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Test	Results
Failed	0
Passed	61
Skipped	0
Total	61

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🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (8/8)	💚
Files	100.0% (8/8)	💚 2.941
Classes	100.0% (8/8)	💚 2.941
Methods	92.174% (106/115)	👍 4.392
Lines	82.877% (1181/1425)	👎 -5.701
Conditionals	100.0% (0/0)	💚