Tweaks to email subject headers

[amyehodge]

There are subject/verb disagreements in a few email subject lines.

FILE: app/mailers/works_mailer.rb

• Line 30: subject = "Reminder: Updates to your deposit in the #{@work.collection_name} collection in the SDR is in progress"
  -- should end with "are in progress"
• Line 39: mail(to: @user.email, subject: "Updates to #{@work_version.title} have been deposited in the SDR")
  -- should end with "has been deposited in the SDR"

FILE: app/mailers/collections_mailer.rb

• Line 73: subject = "Reminder: Updates to your #{@collection_version.name} collection in the SDR is still in progress"
  -- should end with "are still in progress"