Change the way we consider crashes from a given channel in the dashboards

[marco-c]

Currently, we only consider the currently released version in a channel. For example, in the dashboard now we're only showing crashes for '49' when 'release' is selected.

This means that during the few days in the transition between 48 and 49, the graphs will look skewed and it will always seem like there's a spike.

What should we do? I see a few options:

1. Consider 'release' all crash reports which are related to the 'release' channel (this means we will also consider crashes from older versions, e.g. 44, but there shouldn't be many of them).
2. Consider 'release' all crash reports which are related to the 'release' channel and are either in the currently released version (49) or the previous one (48).
3. Keep it like it is now and remember that during the merge days the numbers/graphs might be funky.

@calixteman @lizzard @sylvestre thoughts?