Fix indexing integer timestamp, refactor with timestamp_range

[d-a-bunin]

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

• Add timestamp_range and refactor code
• Add fixes for some bugs

Closing issues

[github-actions]

🚀 Deployed on https://deploy-preview-244--etna-docs.netlify.app

[codecov]

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

❗ No coverage uploaded for pull request base (unaligned-data@7c5aa0e). Click here to learn what that means.

Files	Patch %	Lines
etna/analysis/eda/plots.py	0.00%	4 Missing ⚠️
etna/analysis/forecast/plots.py	33.33%	2 Missing ⚠️
etna/analysis/decomposition/plots.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@                Coverage Diff                @@
##             unaligned-data     #244   +/-   ##
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  Coverage                  ?   90.00%           
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  Files                     ?      198           
  Lines                     ?    13338           
  Branches                  ?        0           
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  Hits                      ?    12005           
  Misses                    ?     1333           
  Partials                  ?        0           

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