The hyperlink and the display name of the branch if the branch is in a folder or the branch name has '#'

This allows the admin to disable the version information about gogs and
go in use in the footer.

- copy link button: #1396, #1168, #1668,
- synxtax highlight: #1712, #1549, #1315, #670
- z-index: #1942

This moves the minimum key sizes into the config file, so that anyone
can modify the restrictions.

"Enable this to disable..." is needlessly confusing, simplify it by being
upfront that this setting disables the Gravatar fetching.