[BLOCKING] Command block minecarts are cleared on relog

Verification builds: Release 1064482
Summary: After exiting and entering the game, command block minecarts lose the command inside. This is an issue in release 1.5 and the current 1.6 build.

Impact: Command block minecarts are completely unusable.

Repro Steps:
1. Open any world
2. /give @p command_block_minecart
3. /give @p rail
4. Place the rail, then the command block minecart atop it
5. Open it and type any text in the "command input" box
6. Exit the world
7. Reopen the world
8. Open the command block minecart again

Observed Results: Text is gone.

Expected Results: Text is still there.

Screenshots/Videos attached: https://streamable.com/uh17g

Notes: I would recommend reaching out to partners and asking whether they have used command block minecarts in their published maps. If so, systems using them are almost guaranteed to be broken, as this bug affects 1.5 retail release.