API: Add send_email method for sending custom emails

[chschenk]

This PR adds the API function send_email which allows sending custom mails to buyers of tickets.

[cla-bot]

Hey there! :) Thank you very much for offering a contribution to pretix! For legal reasons, we need you to sign a Contributor License Agreement in order to be able to merge the code. Sorry for the hassle :( Please download the agreement from https://pretix.eu/about/en/cla and send a signed copy to [email protected]. Feel free to also contact us there or via comments here if you have any questions!

[raphaelm]

Hey, thank you a lot! We'll need to see how we can prevent this from being abused for spamming on pretix.eu, but that's a concern for lager. I added a few notes for improvement.

[codecov]

Codecov Report

Merging #3362 (e86f4cd) into master (b3298c9) will decrease coverage by 0.04%.
The diff coverage is 85.71%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3362      +/-   ##
==========================================
- Coverage   78.05%   78.02%   -0.04%     
==========================================
  Files         417      418       +1     
  Lines       56561    56681     +120     
==========================================
+ Hits        44148    44223      +75     
- Misses      12413    12458      +45     

Impacted Files	Coverage Δ
src/pretix/api/views/order.py	81.76% <83.33%> (+0.02%)	⬆️
src/pretix/api/serializers/order.py	91.98% <100.00%> (+0.06%)	⬆️

... and 55 files with indirect coverage changes

[cla-bot]

Hey there! :) Thank you very much for offering a contribution to pretix! For legal reasons, we need you to sign a Contributor License Agreement in order to be able to merge the code. Sorry for the hassle :( Please download the agreement from https://pretix.eu/about/en/cla and send a signed copy to [email protected]. Feel free to also contact us there or via comments here if you have any questions!

[chschenk]

Hi,
thanks for the review! I have updated this PR accordingly. Please let me know, if there is anything else which i can do to improve this PR.

[chschenk]

Hi, is there anything i can do to move this PR forward?

[raphaelm]

Sorry, this is on me. I never got around to prioritize work on making this abuse-safe for pretix Hosted. I'll look into it again when I have time, thanks for pushing it up the list!