Minor corrections to the form labels in email settings

[AndrewKvalheim]

Checklist

• I have read the Contribution & Best practices Guide.
• My branch is up-to-date with the upstream master branch.
• The tests pass locally with my changes.
• I have added tests that prove my fix is effective or that my feature works (if appropriate).
• I have added necessary documentation (if appropriate).

Short description of what this resolves

Several of the form labels in Manage conference → E-Mails have minor formatting problems, and a couple are missing.

Changes proposed in this pull request

• Add the missing text to labels that were formerly generated by Formtastic.
• Correct malformed labels and Bootstrap form controls.
• Minor copy editing