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.

hc.lib.string.replace only replaces placeholders that appear in the original template. It ignores any placeholders that "emerge" while doing string substitutions. This is done mainly to avoid unexpected behavior when check names or tags contain dollar signs.