Move user specified data to separate table

Right now, the "Ignore update for version X" and "Ignore all updates for this app" are stored in fdroid_app. This means that if a repo is disabled then re-enabled, these preferences are lost. This MR separates out the user specified metadata from the metadata provided by the repository, such that one can change without affecting the other.

For convenience sake, this drops the fdroid_app and fdroid_apk tables rather than migrating them, and then sets a flag in preferences that forces F-Droid to do an index update when started. This is done after migrating already existing user preferences out of fdroid_app and into fdroid_appPrefs.