Add automatic chunk sizing, remove manual chunk sizing

[mpivchev]

Follow up from #2541

Web, Desktop and Android has auto chunking for large files. It's time to add it to iOS as well and remove manual chunk sizing.

1. Add auto chunk sizing:

   • If using cellular, use 10MB chunks
   • If using WiFi, use 100 MB chunks
   • If max chunk count is > 10000 (max count), add + 100 MB to the chunk size to reduce the count. This is an edge case.

2. Remove manual chunk sizing option from settings: