Add a Pause button in the checks list. Fixes #312

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.