Simplify network and Wi-Fi check code.
Schedule syncs independent of Wi-Fi, better method names.

- Ignore Wi-Fi only setting, rather use connectivity state overall. Sync
itself will avoid downloading large data if Wi-Fi only is set.
- Split up forced (immediate) sync and regular sync scheduling methods,
give them different names.
Rename isOnlyUpdateOverWifi() to isLargeDataOverWifiOnly().
Don't download show poster if large data is restricted to Wi-Fi.

- Also properly sanitize poster URL.
Clean up fetch poster task, only runs on allowed connection.

- Don't run and warn user if not on allowed connection.
- Updated to cancel when allowed connection becomes unavailable.
Specific error message if no Wi-Fi available, but we need it.
Use regular network detection where not downloading images.
Only download temporary images if on allowed connection.
Rename settings to 'Images via Wi-Fi only'.

- Use wording familiar from Play apps for summary.
Differentiate between offline and no items in add show screen.

- Use offline empty message if offline.
- Use empty messages for no results or search error with TVDb.
Unify empty view styles.
Separate offline and no items empty message for movie watchlist.
Separate offline and empty message for movies search.
Merge branch 'better-network-detection' into beta