_context.py: Add user_remotes to global user context, the default

of which is set to 'all'

_frontend/app.py & cli.py: Add --user-remotes as a bst main option,
with values of 'all', 'user' or 'none' which when passed will
override the default or user defined context for user_remotes

tests/completions/completions.py: Update for the added flag