Fix weight (priority) of additional repos

Our old DbHelper code would assign all additional repos a weight of 0 which the new database implementation doesn't like. Apps from those repos would simply not appear as if the repo is disabled.

This fix simplifies the weight assignment of initial repos by incrementing the weight for each repo, so the weight assignment works the same way as for manually added repos: later repos get a higher weight.

Incidentally, this also unifies the format of additional_repos.xml and default_repos.xml by removing the weight item from the latter.

Closes #2503 (closed)