Add issues to backlog method added

[jensvog]

Implemented similar to the add issue to sprint method.

[codecov-commenter]

Codecov Report

Merging #902 (e875636) into master (c82c87f) will decrease coverage by 0.01%.
The diff coverage is 16.66%.

@@            Coverage Diff             @@
##           master     #902      +/-   ##
==========================================
- Coverage   36.31%   36.29%   -0.02%     
==========================================
  Files          32       32              
  Lines        6304     6310       +6     
  Branches      971      972       +1     
==========================================
+ Hits         2289     2290       +1     
- Misses       3907     3912       +5     
  Partials      108      108              

Impacted Files	Coverage Δ
atlassian/jira.py	19.18% <16.66%> (-0.02%)	⬇️

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