Add lets self fix subcommand for config migrations

[kindermax]

Having this command will simplify migrations from old to new config formats.
We can use it for example for this task #304

usage:

• lets self fix --dry-run will show changes as diff
• lets self fix will update all lets configs including mixins (excluding remote mixins, but will report that remote mixins also must be updated)

Maybe tell user that he uses deprecated features on lets invocations. Add deprecations section to docs