compliance with the principle of DRY in the email.py file

[mojtabaakbari221b]

file: https://github.com/sunscrapers/djoser/blob/master/djoser/email.py

In the get_context_data() method of three of the classes, many lines of code are repeated. (Dont Repeat Yourself)
If we want to add a line of code, we have to do this 3 times, it is definitely difficult to maintain this code.
I wrote a mixin in the email_mixins.py file and put these duplicates in it and inherited the desired classes from it.
Also, the common part of the code that caused this to be repeated 3 times was the different context["url"], so I solved this problem by adding a parameter to the class.

[codecov]

Codecov Report

Merging #689 (3c68325) into master (82c837e) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #689      +/-   ##
==========================================
- Coverage   99.02%   99.01%   -0.01%     
==========================================
  Files          26       27       +1     
  Lines         820      814       -6     
==========================================
- Hits          812      806       -6     
  Misses          8        8              

Impacted Files	Coverage Δ
djoser/email.py	100.00% <100.00%> (ø)
djoser/email_mixins.py	100.00% <100.00%> (ø)

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