fix(cli): Resolve problem with assert-invariants cmd

[pirtleshell]

Description

The assert-invariants command works by initializing a TestApp with the genesis file state, which runs the invariant checks and InitGenesis. When we added a validator to all TestApp instances in #1477, we began modifying state of the genesis being checked in assert-invariants.

This PR makes adding the validator optional so that assert-invariants checks only state in the actual genesis file.

Additionally, removes unused no-op migrate command and moves the assert-invariants command into the cmd directory.

Checklist

• Changelog has been updated as necessary.

[nddeluca]

looks good ty for the fix