ContactForm: Selected default channel element must be required

[sukhwinder33445]

For example, When Rocket.Chat is selected as the default channel, the Rocketchat username element must be required.

[julianbrost]

Note that it might make sense to wait with this until we've got more of a plan regarding more flexible channel plugins. At some point, it should be possible to add some (possibly third-party) channel implementation, let's say for the Signal messenger as an example. Ideally, that plugin will be able to advertise that it needs a mobile phone number as a contact address and then reuse this attribute (rather than having address fields for SMS phone number, Signal phone numbers and five others for messengers that take a phone number as an address).

[nilmerg]

This is only a small bit of code to achieve this, once it's there we can still change it in case channel plugins behave like this.

[nilmerg]

Note that this is currently only possible for known channels. (i.e. what Icinga Notifications provides out of the box)