Move server specific features to account settings

[boothj5]

For configuration settings that depend on the server supporting certain features, the setting should be associated with the account, rather than globally for all accounts,

Example: #933 If one server supports ping, but another doesn't and both accounts are logged in, profanity should only disable autoping for the account on which it is not supported.

Another example: /carbons When carbons are enabled globally, the error message is shown on login: Server error enabling message carbons: service-unavailable for any accounts for which the server doesn't support carbons.