Cache in footer view

[stsewd]

The footer view is assessed for each page view. We should make it faster :)

Ref: #6125 (comment)

[humitos]

Cache keys should be improved to avoid potential conflicts: https://github.com/readthedocs/readthedocs.org/pull/6125/files#r320606597

[tapaswenipathak]

Hi folks: I have added few PRs, let me know your thoughts #6148 (comment).

[dojutsu-user]

@stsewd I think it will result in not having correct results for #6121. We are storing the page views count from the footer views. So if that is cached -- the page view count will not be correct.

[stsewd]

We are going to move page views to its own endpoint #7739, after that we are going to cache it trough cloudflare.