Add option to send e-mail to confirmed speakers without any slides.

[Quaz212]

This PR should fix issue #432 to add an option to send an e-mail to all confirmed speakers without any uploaded slides/resources.
Please double check the filter I used in views/mail.py, I set it to also match state='confirmed' as otherwise rejected submissions would get e-mailed too.

How Has This Been Tested?

Existing tests still pass, except two which failed before I made my changes. No additional tests have been added.
Manually tested this by creating a test event with submissions, etc. in the dev environment.

Screenshots (if appropriate):

n/a

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.
• My change is listed in the CHANGELOG.rst if appropriate.

[codecov]

Codecov Report

Merging #783 into master will decrease coverage by 0.02%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #783      +/-   ##
==========================================
- Coverage   89.29%   89.27%   -0.03%     
==========================================
  Files         140      140              
  Lines        8740     8742       +2     
  Branches     1146     1147       +1     
==========================================
  Hits         7804     7804              
- Misses        641      642       +1     
- Partials      295      296       +1

Impacted Files	Coverage Δ
src/pretalx/orga/forms/mails.py	88.15% <ø> (ø)	⬆️
src/pretalx/orga/views/mails.py	88.32% <0%> (-0.7%)	⬇️

[rixx]

Sorry for the delay in merging – I had somewhat hoped to get round to replacing the current unsatisfactory email editor, but that's going to have to wait. Thank you very much for your contribution!