Entering secret key in "Join Account" should not be possible

[ebma]

It is possible to enter a secret key into the "Multi-Signature Account" text field when using the "Join Account" feature.
This will however result in network problems because Solar is doing network requests for accounts using a secret key.
The user is also not able to resolve this issue except for wiping the whole application data because clicking on this invalid account will only show the network error without giving an option to delete the account.

Expected behavior:
The user should not be able to enter a secret key into the text field when trying to join an account. An appropriate error message should be shown above the text field instead.