pleroma_ctl: Run mix tasks using rpc instead of eval

pleroma_ctl: Run mix tasks using rpc instead of eval

This patch changes pleroma_ctl to call into a running instance instead of spinning up
a new one for mix task execution. This significantly decreases the time needed
for mix task execution (on my machine for `pleroma_ctl user set rin --no-admin`
the difference is 0.2s vs 4.2s) and allows mix tasks to affect the
instance context, for example, this allows to clear the cache after user
deletion