Allow user to edit number of update history days in prefs.

Instead of using hardcoded 14 days.
When deleting repos, mark all other repos for fresh re-fetch.

Consider following scenario: there's repo A with ETag support. Now
there's repo B which shadows some APK from repo A. If repo B then deleted,
info for the APK will be gone, and A's verion won't be fetched because of
ETag caching. So, we need to force re-fetching indexes from all repos
in such case.