Improve error handling if no valid channel could be determined

[ruKurz]

1. If a user with the role config-expert logs in and a default channel is configured that does not exist or cannot be reached, the user (config-expert) should be notified that he needs to change the default channel setting. Optionally, in this case, the config expert could be offered the option to select the default channel directly, so that he can fix the problem directly in the situation in which it occurs.

2. If an end user cannot assign a channel to a support request and the default channel does not exist, the user should be informed that the support request could not be delivered (possibly with a reference to the corresponding config-expert).

[jschirrmacher]

As a first step, I've improved the search for a default channel (in fact, it didn't search at all before...), so that a config-expert can simply use the configuration screen to add a default keyword to one of the channels.

If the config-expert uses the default keyword more than once, the first one found is used.
if no default keyword is assigned, the search function falls back to using the first channel which has the default property set. This properties identifies channels that users are joined by default.
If even such a channel cannot be found, it falls back to 'general', but ignores character case now, so that 'GENERAL' would also be found.

These improvement doesn't solve the problem of error handling in general, but imho this is at least a valid solution for the MVP. What do you think, @ruKurz ?