MICROBIN_ENABLE_BURN_AFTER is not default=false

**Describe the bug**
I trusted the comments in .env, and they seem to be wrong.

**To Reproduce**
Don't set a value to `MICROBIN_ENABLE_BURN_AFTER`.

**Expected behavior**
`MICROBIN_ENABLE_BURN_AFTER=false` as per comments in .env.

**Screenshots**
Well, "Burn after" was enabled.

**Additional context**
After manually inserting `MICROBIN_ENABLE_BURN_AFTER=false` in .env, "burn after" was disabled.
