Using Jinja2 templating actually simplifies slightly the code, but, most importantly, enables a much greater flexibility
to customize the notifications' content.
This change is breaking: the tokens like `{base_url}` should instead be written s `{{ base_url }}`. It would be very easy
to make it non-breaking, but I think it would be preferable to stick to the default Jinja2 syntax. To make it non-breaking,
the Jinja2 environment should be configured this way:
```
jinja2_env = Environment(
loader=BaseLoader,
variable_start_string='{',
variable_end_string='}',
)
```
NB: This change could enable a few more follow-up enhancements:
* It could make sense as a later evolution to implement the same for the URL list. This could enable features like:
- Customizing the sender based on the watch parameters through global settings
- Sending the notification to a different email/channel/group based on a watch attribute, like its tag
* Jinja2 plays fine with complex variables. It would be relatively easy to expose many more variables that could be used
in the notification.
Changes
=======
* Use Jinja2 template rendering to parse tokens in the notification body and title.
* Replace the settings validator to check that the body and title are both valid Jinja2 templates
and that they do not contain any invalid token/variable.
* Update the corresponding documentation, setting pages and tests.
