861 bug ssh key removal

Allow user to remove all ssh keys.
Prohibit user to add key with content, that already present in one of his keys.
Prohibit user to create session if he has no ssh key.

https://bugs.tddium.com/bugzilla/show_bug.cgi?id=861

Depends on https://github.com/solanolabs/tddium_site/pull/900