prettify the "all events feed" banner

[ionous]

as per this from @carrythebanner re: the new all events feed.

#797 (review)

Could also add the cal feed banner to the /calgrid page, and could probably hide it on individual ride listings...
With 2 promo banners stacked there's a bit much in terms of top matter, but we'll be hiding the PP promo banner in 2 days so that's a non-issue IMO.

the feed info is hidden from single events:

but we could add it to the calgrid, and we should figure out how to make it play nicer with the pedalp banner.

( some possible ideas: maybe move it to the bottom, merge it with the "safe for all" section, maybe add info about the bikefun app? )