Fixes: #281

In SMS, Signal and WhatsApp forms, reject the form if
user unchecks both "alert when check goes DOWN" and
"alert when check goes UP".

Fixes: #202

The PhoneNumberForm is used in "Add SMS" and "Add Phone Call" pages.
The PhoneUpDownForm is a subclass of PhoneNumberForm and
adds "up" and "down" boolean fields. It is used in "Add Signal"
and "Add WhatsApp" pages.

The existing 40 character limit prevents using the integration will
Matrix servers that might have a fairly lengthy hostname (ie.
'matrix.internal.example.com' would only allow 12 characters for the
room name or ID, and room IDs are 19 characters long).

Increasing the `max_length` to `100` is still fairly arbitrary but it
does match up with the `max_length` of the `name` field of the `Channel`
model and should cover most instances.

.Webhooks can have different request bodies and headers for "up" and "events".

header storage.

services like https://ifttt.com/maker_webhooks which require a
specific content type, like application/json.