fix: remove position validation from post entities and references

[RiccardoM]

Description

This PR removes the position validation from within a post's entities. This is done to make sure that DApps that store the post's text on other places (i.e. IPFS or any external storage) can use the tags properly.

Before, it wasn't possible to use a tag specifying start and end values that where exceeding a post text length. But for posts that store their text on other storage solutions, it doesn't make sense to check for this. Instead, allowing to specify arbitrary values for start and end indexes allows to support these cases as well.

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• confirmed the correct type prefix in the PR title
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[codecov]

Codecov Report

Merging #941 (019f147) into master (8adb7c9) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #941      +/-   ##
==========================================
- Coverage   82.76%   82.74%   -0.02%     
==========================================
  Files         166      166              
  Lines       14014    14003      -11     
==========================================
- Hits        11598    11587      -11     
  Misses       1943     1943              
  Partials      473      473              

Impacted Files	Coverage Δ
x/posts/types/models.go	76.84% <ø> (-0.64%)	⬇️

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