added settings to customize activation mail sending

[Chadys]

I think applications should be able to customize the users' activation procedure for their use cases. In the current version, when you want activation upon registration you also get activation upon email and username change which might not be desired. I added two settings, ACTIVATION_NEEDED_ON_EMAIL_UPDATE and ACTIVATION_NEEDED_ON_USERNAME_UPDATE that enable to customize on what event an activation is needed. They are both set to True by default, so as to keep current behaviour. The SEND_ACTIVATION_EMAIL documentation was also missing the activation on username change.

[codecov[bot]]

Codecov Report

Merging #401 into legacy will not change coverage. The diff coverage is 100%.

@@           Coverage Diff           @@
##           legacy     #401   +/-   ##
=======================================
  Coverage   97.71%   97.71%           
=======================================
  Files          19       19           
  Lines         699      699           
=======================================
  Hits          683      683           
  Misses         16       16

Impacted Files	Coverage Δ
djoser/conf.py	96.87% <ø> (ø)	:arrow_up:
djoser/serializers.py	96.82% <100%> (ø)	:arrow_up:
djoser/views.py	99.27% <100%> (ø)	:arrow_up:

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[BrendanMyers88]

Hey, this PR looks like it was force-pushed to the master branch and then the base was moved to legacy. I couldn't find the code that was changed on the master branch. Is there any chance of this getting merged in soon?

Thanks!

[Chadys]

Hello, I made this just before version 2.0 came out and it was too late to include my changes in a last 1.X by then. I need to update this PR to become compatible with current djoser version, but I didn't have anymore time to do it then and I still don't have now. Feel free to fork my work and propose your own, up-to-date, PR for that feature. Else I can't guarantee when I'll have enough free time to get back to it.