Migration and genesis validation fixes

[RiccardoM]

Description

• Removed the block-interval flag from the migrate command
• Changed the migrate command to use v0.15.0 instead of stargate, for consistency
• Changed v0.15.0 migration types to use the current types
• Changed all PostID to PostId and ParentID to ParentId
• Fixed genesis validation errors

Checklist

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

[codecov]

Codecov Report

Merging #340 (018b98e) into master (8eafbff) will increase coverage by 0.00%.
The diff coverage is 84.69%.

@@           Coverage Diff           @@
##           master     #340   +/-   ##
=======================================
  Coverage   82.02%   82.03%           
=======================================
  Files          99       99           
  Lines        3756     3758    +2     
=======================================
+ Hits         3081     3083    +2     
  Misses        564      564           
  Partials      111      111           

Impacted Files	Coverage Δ
x/posts/client/cli/query.go	61.02% <0.00%> (ø)
x/posts/keeper/grpc_query.go	0.00% <0.00%> (ø)
x/posts/legacy/v0.15.0/types.go	0.00% <0.00%> (ø)
x/profiles/legacy/v0.15.0/types.go	100.00% <ø> (ø)
x/relationships/legacy/v0.15.0/types.go	0.00% <ø> (ø)
x/reports/legacy/v0.15.0/types.go	0.00% <0.00%> (ø)
x/posts/types/posts.go	89.61% <57.14%> (ø)
x/posts/types/msgs.go	96.35% <86.66%> (ø)
x/posts/keeper/msgs_server.go	95.23% <96.66%> (ø)
x/genutil/legacy/v0.15.0/migrate.go	100.00% <100.00%> (ø)
... and 9 more

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