key backup: Add a dedicated section to settings

[manuroe]

@manuroe commented on Mon Jan 21 2019

See details in the meta issues.

[bmarty]

New section in settings, between Cryptography and Devices:

Cryptography keys management

Manage keys recovery (new item)

Export E2E keys (move existing item here)

Import E2E keys (move existing item here)

[bmarty]

Done in #2908