Create a classifier to detect regression bugs that have no regression range

[marco-c]

Needed for mozilla/bugbot#221.

To do this, we'll need to collect some labels.
We can automatically create positive labels by getting bugs that have the regression keyword and the regression-wanted keyword.
We can't automatically create negative labels because we can't be sure that the regression range is available, when we have the regression keyword but no regression-wanted.

[ayush-1506]

@marco-c I reckon we'd have to collect negative labels manually? How should I be doing this? When I visit a bug, what should I look for to add it into the negative category?

[marco-c]

We actually have a 'Has Regression Range' field, so we can collect some positive and negative examples using that (similarly to 'Has STR').