Fix footer does not stick to the bottom

[simoarpe]

Resolves #576

This PR does not affect the current behavior of the footer except for the edge case described in the issue: when there is a small content or none the footer now stays at the bottom the page.

Because the height of the footer can change, most of the solutions around didn't work. Using flexbox for a sticky footer not only doesn't require any extra elements, but allows for a variable height footer.

[daattali]

Thanks for the PR!

This changes the layout of the entire page. Since this theme is used by tens of thousands of people, I'm extremely cautious in making such changes. I would like to see this tested with different config parameters, to make sure it doesn't break any existing features. How much have you tested this outside of your own project?

[simoarpe]

I ran some tests, but probably not as much as it deserved.
Tried on different browsers, desktop and mobile.

[O-Mutt]

Might be cautious of flex-direction with FF https://caniuse.com/#search=flex-direction

[daattali]

Thank you @Mutmatt

Have you tried this solution to see what it looks like on FF? I just rewrote the majority of the HTML and CSS while upgrading to bootstrap 4, so now seems like a good time to test this as well.

[daattali]

@simoarpe the theme recently underwent huge changes, updating to bootstrap 4 which uses flexbox, so if you could update this PR to work with the current codebase, I'd be happy to accept this

[daattali]

Closing this PR due to inactivity. If anyone wants to revive it in the future, feel free to submit a new one!