archives: use separate table

Cleaner to separate things from the backups table.

* icon, appConfig, appStoreIcon etc are only valid for archives
* older version cloudron does not have appConfig in backups table (so it
  cannot be an archive entry)

src/test/backups-test.js

@@ -31,8 +31,6 @@ describe('backups', function () {
         format: 'tgz',
         preserveSecs: 0,
         label: '',
-        archive: false,
-        appConfig: null
     };
 
     const appBackup = {
@@ -48,8 +46,6 @@ describe('backups', function () {
         format: 'tgz',
         preserveSecs: 0,
         label: '',
-        archive: false,
-        appConfig: null
     };
 
     describe('crud', function () {