2-weekly moving average on epicurves

[zkamvar]

[see: https://github.com/reconhub/incidence/issues/105] on epicurves - possible to add a 2 weekly moving average? - issue posted on recon. Long term issue case.

Originally posted by @aspina7 in #185 (comment)

[aspina7]

either wait for incidence2 or consider adding additional code using {slider} to then swap out at a later date

[amygimma]

the rolling average code looks like it's only going to be on this helper package, which isn't on CRAN ... should we use something else (prob slider) in that case?

I can pick this one up when we decide on that

[aspina7]

think we are better off using one of the options from the epirhandbook chapter - agree slider is probably best option

[amygimma]

cool i can start on this one then. it's a straightforward one to get me used to the code base

[aspina7]

done for all outbreak templates in #275