Implement forecast decomposition for DeadlineMovingAverageModel

[brsnw250]

Before submitting (must do checklist)

• Did you read the contribution guide?
• Did you update the docs? We use Numpy format for all the methods and classes.
• Did you write any new necessary tests?
• Did you update the CHANGELOG?

Proposed Changes

Closing issues

closes #1171

[github-actions]

🚀 Deployed on https://deploy-preview-1186--etna-docs.netlify.app

[codecov-commenter]

Codecov Report

Merging #1186 (18495ce) into master (9483e6a) will decrease coverage by 0.28%.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##           master    #1186      +/-   ##
==========================================
- Coverage   87.67%   87.40%   -0.28%     
==========================================
  Files         177      177              
  Lines       10408    10438      +30     
==========================================
- Hits         9125     9123       -2     
- Misses       1283     1315      +32     

Impacted Files	Coverage Δ
etna/models/deadline_ma.py	98.66% <100.00%> (+3.66%)	⬆️

... and 3 files with indirect coverage changes

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