Display Map Skew Source

We need to add a column to the table on the first tab of the Shiny App which displays the citation (just a doi link) of the regional skew and standard error if the "Use Map Skew" option is selected for a site.

I've modified the read_skew_map_pt() function so that it now returns a dataframe column, 'Citation' containing the appropriate citation for the regional skew value. If the "Use Map Skew" option is selected for a site, the value from the citation column should be displayed in a new column called "Map Skew Citation" in the table on the first tab of the app. The populated citations should also be hot-linked to the citation url. This column might end up being obnoxiously wide, so hopefully we can find some way to handle that in a relatively nice manner.