Refactor Mix tasks

Refactor Mix tasks

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.