upgrades: Preparing for dist-upgrade and opt-out

• Prepare to handle backports related to #1644 (closed), where a new version of backports may become available later.
• Move backports security notice to Security page.
• Enable backports on testing in development mode only.
• Remove periodic check to enable backports. Backports are still enabled during setup on stable and testing+develop. Prepares for #1855 (closed).
• Split apt preferences into 2 files: one for freedombox, and one for apps. freedombox is always set to current release's backports, but the apps file entries are specific to a release.

When backports can be activated for the current release (note that most users won't see this):

After activating backports for the current release:

Tests:

• Ran functional tests for upgrades, security, and help on testing/unstable.
• Ran "upgrades setup" and "upgrades setup-repositories" actions on testing/unstable.
• Activated backports on testing in develop mode.
• Additional tests listed per commit.

Next steps are to actually implement #1855 (closed) and #1644 (closed).

Signed-off-by: James Valleroy jvalleroy@mailbox.org