feat: add COMMENT_CONTENT permission

[RiccardoM]

Description

This PR adds the support for the COMMENT_CONTENT permission within the x/posts module.
This permission allows only commenting existing content, without being able to create new root posts. This is particularly useful to all applications that use subspace sections to represent personal blogs: in this case you want to have only one user with the WRITE_CONTENT permission, while external users should only have the COMMENT_CONTENT permission instead.

This feature request was raised by @g-luca for Scripta.

Closes: #997

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

Codecov Report

Base: 80.50% // Head: 80.50% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (0006dda) compared to base (974fc37).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #998   +/-   ##
=======================================
  Coverage   80.50%   80.50%           
=======================================
  Files         175      175           
  Lines       15289    15291    +2     
=======================================
+ Hits        12308    12310    +2     
  Misses       2450     2450           
  Partials      531      531           

Impacted Files	Coverage Δ
x/posts/keeper/msg_server.go	89.91% <100.00%> (+0.05%)	⬆️

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[manu0466]

Looks good to me!