fix(avatar): handle nil email with fallback to member-ID-based placeholder

[mroderick]

This was re-introduced when we null'ed invalid email addresses in the database in recent work fixing sending out email invites for workshops.

Summary

• Fixes ActionView::Template::Error: undefined method 'strip' for nil when rendering avatar for members with nil email
• Extracts MD5 hashing into new DigestHelper#md5_of helper (pure function)
• Refactors Member#avatar to use fallback email (member-{id}@example.com) when member.email is nil
• Removes deprecated Member#md5_email method

Error

Refs: https://app.rollbar.com/a/codebar-production/fix/item/codebar-production/676

ActionView::Template::Error: undefined method 'strip' for nil
Member#md5_email(/app/app/models/member.rb:176)
    Digest::MD5.hexdigest(email.strip.downcase)
Member#avatar(/app/app/models/member.rb:102)
    "https://secure.gravatar.com/avatar/#{md5_email}?size=#{size}&default=identicon"

Testing

• Added tests for md5_of helper in spec/helpers/application_helper_spec.rb
• Added test for nil email fallback in spec/models/member_spec.rb
• All tests passing:
  • 41 member spec tests
  • 10 application helper spec tests