Add Snapshot Restore command

[jac]

Snapshot Restore generates commands to restore dumped snapshots to a Sourcegraph instance

Usage:
After generating a dump the restore commands can be generated with

src snapshot restore <docker|kubectlpg_dump>

The --run command can be used to attempt to automatically run these commands. Useful e.g. in the AMIs e.g.

src snapshot restore --run <docker|kubectlpg_dump>

• Changed snapshot args ordering
  • To get the --run flag to work it was necessary to change arg ordering as Go's flag package requires all flags come before non flag args

src snapshot databases docker --run
# changes to
src snapshot databases --run docker

• Removed the -t flags from <docker|kubectl> exec
  • afaik only -i should be necessary for input/output redirection. Without -t we get better compatibility (prevents device is not a TTY error)

Test plan

• Generated snapshots using the snapshot tool and then restored them to another instance (running the same Sourcegraph version)

[jdpleiness]

Looks good! 🚀

[bobheadxi]

nice work! a few comments