Implemented the post message cap to 500 characters

[RiccardoM]

Description

This PR allows to cap the post message length to 500 characters, fixing #67

Checklist

• Targeted PR against correct branch.
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Wrote tests.
• Added an entry to the CHANGELOG.md file.
• Re-reviewed Files changed in the Github PR explorer.

[codecov-io]

Codecov Report

Merging #70 into master will increase coverage by 0.28%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #70      +/-   ##
==========================================
+ Coverage   75.36%   75.65%   +0.28%     
==========================================
  Files          17       17              
  Lines         759      768       +9     
==========================================
+ Hits          572      581       +9     
  Misses        176      176              
  Partials       11       11

Impacted Files	Coverage Δ
x/posts/internal/types/msgs.go	100% <100%> (ø)	⬆️
x/posts/internal/types/post.go	95.65% <100%> (+0.24%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a0574ed...a092352. Read the comment docs.

[leobragaz]

Seems to be all ok, i've check even if the vars in fmt.Sprintf() matches the correct places.

[RiccardoM]

I'd like to have @kwunyeung reviewing this PR too if possible 😄

[kwunyeung]

LGTM