Header and Footer Improvements

[dotcomaki]

@oohwooh's suggestions:

• I think adding profile pictures would be nice, like we have in the similar footers on a lot of other CodeDay sites.
• Some way to cache the data might be helpful. I don't know the best way to implement this but as the number of files grow, it makes it more and more likely we'll hit API limits (and even without api limits, it's still a bunch of unnecessary requests)
• I think the last modified date should be under the page header, in the footer it's easy to miss and sometimes it can be really relevant. It would be really frustrating to spend x amount of time reading a page, get to the bottom and find out it's 5 years old and probably no longer relevant.
• Knowing who specifically made the last contribution I don't think is very helpful information. We could sort the list of authors by their last contribution to still have this information present, without needing to state it twice.