[iOS] Add ability to open and download currently unsupported videos

[sergeymomot]

Some videos are not open in the app (only “Open in browser” button is shown)

We need:

• find out the reason why these videos are not open
• find out how to download this content for offline mode

[marcotuts]

There are some videos that can't be downloaded based on format - one example is Youtube videos / embedded videos like that. fyi @nizmaylova

[marcotuts]

Some other videos can be configured to be unavailable on mobile entirely, so those won't be able to be viewed or downloaded. there is a "view on mobile" configuration field that is now respected / used in the app based on a PR. Educators can decide if a video isn't available in this way.