support Universal links to create https:// "open-in-app" pages

[roelandp]

Concept: One links to a steemwallet.app hosted page consisting of a steem://-uri spec formatted url and if on mobile with steemwallet.app installed, the app would instantly open and the receiver check's out.

• This way anyone can send actual links beyond just scanning qr-codes or just using a steem:// uri (which might return a "don't know how to handle that link").
• Should a user not have the app s/he can be nudged to install or checkout via other steem://-uri receptive apps / steem keychain / steemconnect

This needs some work on both the app's for ios & android:

• iOS: Universal Links docs
• Android: Applinks docs