Timeliness table doesn't currently indicate if publishers started publishing in the last 12 months

[Bjwebb]

@Mattdevinit was asking for this feature. I won't have time to implement it, but leaving it as an issue for someone else to pick up.

The first_published value at https://github.com/IATI/IATI-Dashboard/blob/master/timeliness.py#L50 needs to be passed through to the template and used somehow.

We might want a simple column of the dates that publishers started publishing, or we might want to do something different to the zeroes from before they started publishing (this might be muting, but currently all zeroes are muted).

[hayfield]

(related issue is marked as enhancement)