Provide a front page for /compare

[exap]

At the moment there are pages like
/user/repo/compare/branch-a...branch-b
but the page
/user/repo/compare
doesn't exist.

Github supports this front page. It'd be nice if Gitea would support this also.

I use Gitea 1.1.0

[gsantner]

Having an additonal "Compare" button (or/and a "changes since this commit") while viewing a commit would also be great

[lunny]

I think this is duplicated with #1352. Let's discuss continue there.