gluon-autoupdater: unconditionally perform an update 5 minutes after boot when the whole PRIORITY delay has passed

The probability field is removed, updates are performed once per night between
04:00 and 05:00.

All announce.d scripts have been moved to /lib/gluon/announce/announce.d
The script /lib/gluon/announce/announce.lua will collect all information
and output json.

This also fixes the type of a few fields which were strings instead of numbers.

It is now possible to specific multiple branches like this:

config branch stable
  option url ...
  list pubkey ...
  ...

config branch testing
  ...

and select one in autoupdater.settings:

config autoupdater settings
  option branch "stable"