build(deps): bump nodemailer from 6.4.5 to 6.4.11 in /backend

Sourced from nodemailer's changelog.

6.4.11 2020-07-29

• Fixed escape sequence handling in address parsing

6.4.10 2020-06-17

• Fixed RFC822 output for MailComposer when using invalid content-type value. Mostly relevant if message attachments have stragne content-type values set.

6.4.7 2020-05-28

• Always set charset=utf-8 for Content-Type headers
• Catch error whn using invalid crypto.sign input

6.4.6 2020-03-20

• fix: requeueAttempts=n should requeue n times (Patrick Malouin) [a27ed2f7]

6.4.4 2020-03-01

• Add options.forceAuth for SMTP (Patrick Malouin) [a27ed2f7]

6.4.3 2020-02-22

• Added an option to specify max number of requeues when connection closes unexpectedly (Igor Sechyn) [8a927f5a]

6.4.2 2019-12-11

• Fixed bug where array item was used with a potentially empty array

6.4.1 2019-12-07

• Fix processing server output with unterminated responses

6.4.0 2019-12-04

• Do not use auth if server does not advertise AUTH support [f419b09d]
• add dns.CONNREFUSED (Hiroyuki Okada) [5c4c8ca8]

6.3.1 2019-10-09

• Ignore "end" events because it might be "error" after it (dex4er) [72bade9]
• Set username and password on the connection proxy object correctly (UsamaAshraf) [250b1a8]
• Support more DNS errors (madarche) [2391aa4]

6.3.0 2019-07-14

• Added new option to pass a set of httpHeaders to be sent when fetching attachments. See [PR #1034](nodemailer/nodemailer#1034)

6.2.1 2019-05-24

• e3055c4 v6.4.11
• 3cfbf60 Create pull_request_template.md
• 35f9e33 Create CODE_OF_CONDUCT.md
• 8985bde v6.4.10
• a0cab71 v6.4.8
• 5874437 Updated testable node versions
• 643c039 v6.4.7
• b039ca5 Force charset for Content-Type headers even if not needed
• 5702a71 catch errors from invalid crypto.sign
• 5e00fb5 Update issue templates
• Additional commits viewable in compare view

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