1. Move Mix tasks into a `pleroma` namespace, to avoid
collisions with dependent packages.
2. Rename and condense tasks into two `pleroma.user` and
`pleroma.gen.instance` tasks for consistency with Hex and Phoenix.
3. Add additional functionality to the tasks to make them more
user-friendly. Arguments with sensible defaults were demoted to flags
and in the interactive `generate_config` (renamed to
`pleroma.gen.instance`), flags were added to allow non-interactive
use, though interactive use remains the primary interface. That task
also now prompts the user for database parameters.
4. Documentation has been added to both tasks such that `mix help` now
shows useful information.
5. Finally, use of IO.puts in tasks has been replaced with Mix.shell()
equivalents to make the behavior more consistent with Mix tasks in
other packages, and such that variables like MIX_QUIET are respected.
The only exception is in `mix pleroma.user reset_password`, wherein
the URL must always be printed regardless of the value of MIX_QUIET
since that's its entire purpose.
