Bug 25672: Convert to restricting to listed repositories only

Bug 25672: Convert to restricting to listed repositories only

I think instead of a plain on/off switch we should use it in combination
with the plugin_repo's and set it to restrict to only those repos' (i.e.
disable uploads entirely if no repo's are listed, or just allow those
repo's when there are).

This patch achieves that, but only if plugins are installed via the
plugin browser method. We disable all direct upload avenues, so install
is blocked for other cases.

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: David Cook <dcook@prosentient.com.au>
Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>