Add a model to identify invalid reports for the new Firefox in-product reporter

[ksy36]

The new reports will contain url, description (optional) and category (optional).

To train the model we could use issues data from https://github.com/webcompat/web-bugs/issues to start and use only
Steps to Reproduce part from the report.

For example, webcompat/web-bugs#129079:

Steps to Reproduce:
Website icons not loading, it happens all over the site, it is best seen by accessing "Login în with browser". They appear correctly on Chrome.

Some reporters provide additional description (when it doesn't fall into a specific category)

webcompat/web-bugs#128961

Description: Couldn't scroll down
Steps to Reproduce:
Scroll down the page, then scroll to top, scroll down again, the page couldn't scroll (will always back to top).

In this case, we can probably combine these together, it could be something like: "Couldn't scroll down. Scroll down the page, then scroll to top, scroll down again, the page couldn't scroll (will always back to top)."