- We use the LOCALHOST key for testing
- Takes this opportunity to straighten a bit the way we create import.

We might want to do that in the future but for now, it is basically
unused code.

The goal is to make the function more usable in web hooks tests.
We now have a function to check
and one to compute a signature.
It makes also fixtures independent with regards to content change.

Only one case is not tested for now, when basically we are
communicating with GitHub.
There are a couple of strategies for mocking the service. It needs a
bit more cycles of thinking.

- Removes the GET. Flask handles this already for us
- Creates only 1 requests for setting labels instead of two, probably
avoiding race condition mentioned in webcompat#1553
- Sends plain/text messages where it is granted.
- Logs the failure when tagging didn’t happen properly
- Renames set_label as set_labels and rationalises the functions