Cannot sign in with telegram accounts having two-factor auth activated.

[tallero]

When two-factor auth is activated, telegram-purple needs

<setting name='password-two-factor' type='string'>PASSWORD</setting>

in accounts.xml to sign in. Actually the only way I found to do this is manually editing the file on the server so at the moment two-factor users can not register to the transport.

Possible solutions I thought of:

• add a form that picks the value of this field on the registration screen (would this be client indipendent? does it break transport standard?)
• add the capacity to edit accounts.xml for purple backend in settingsadhoccommand.cpp
• adding support for "purple request API", so that we can use normal telegram-purple solution.way of retrieving the password.
• adding an exception in telegram-purple for clients not supporting "purple request API" not only for request_code but also for two auth password.

[vitalyster]

Spectrum is able to forward purple input requests, but currently it needs to be:

1. explicitly whitelisted by primary string or another parameter, see examples
2. token should be saved in spectrum database on successful sign in
3. and restored on next login request when Spectrum recreates accounts.xml from its database.

[vitalyster]

Duplicate of #264