Allow all users to add two extensions.

- Display upgrade hint if two extensions are already added.
Push version to 13.1, 11256.
Change log for 13.1.
Add new strings to all languages.
Avoid crash on null database return.

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https://crashlytics.com/seriesguide/android/apps/com.battlelancer.seriesguide/issues/534fda6ce3de5099ba170f6c
Merge branch 'release-13.1'