Extensions on mobile platforms

[alex-tsx]

At the moment Beacon UI does not expect to have extensions installed, when navigator.userAgent is of mobile nature. It does check their presence (pings are received by extension), but will not provide corresponding user-actions (render corresponding buttons).

Thus, there is no attempt to connect with extension at all. Noticed this, while adapting our wallet extension for mobile-first browsers.

With this consideration, UI is up to be decided, but desired effect is reflected by this PR.