Trying this on for size for a while.

The commands `omz plugin {enable,disable}` and `omz theme set`
automatically reload the zsh session on success. With this
change, the CLI checks whether the commands are run in an
interactive session before reloading the zsh session.

This change also conditionally sets the completion function
for `omz` so that it's not done in a non-interactive session.

Co-authored-by: C. Yavuzsen <cyavuzsen@comscore.com>

BREAKING CHANGE: the `zsh_reload` was deprecated long ago and showed
a deprecation message. The `src` function was replaced by the CLI
command `omz reload`, so use that instead from now on.

BREAKING CHANGE: the `django` plugin was deprecated in 2021-09-22. With
this change it has now been removed altogether. Zsh already provides
built-in completion for Django commands.

Co-authored-by: Matthias Doering <matthias.doering@teamix.de>

BREAKING CHANGE: the `npx` plugin used a feature of `npx` to check for
npm packages and run them if a command was not found. This feature was
removed in v7.0.0 and was deemed insecure. The `npx` plugin is now
officially deprecated and will be removed soon.

Fixes #10452

Fixes #9737

Fixes missing $USER value in ArchLinux sh (bash)

Co-authored-by: Marc Cornellà <hello@mcornella.com>

Closes #4962

Closes #6555

Co-authored-by: Mike Mattice <mmattice@reliant.io>