Add a link to the opam file on the package overview page

[tmattio]

Closes #755

[sabine]

Might need better contrast, icon size / stroke width of icon is different to the icon of the repository link.

LGTM otherwise.

[tmattio]

Thanks, I updated the stroke width. The contrast is the same as the other links, so I think we can keep it like this?

[sabine]

Icon size is inconsistent with the icon above which is used in a similar role.

Other than that, I think it's okay.

[cuihtlauac]

BTW, do we need the arrow-top-right-on-square? I don't get the point

[tmattio]

Ok, the links in the overview are getting messy and inconsistent, so I did some work to make it more consistent.
Here's the result:

And when the documentation fails:

[cuihtlauac]

Much nicer!

[cuihtlauac]

Me being cheeky: Shouldn't the source tarball link receive the same treatment?

[tmattio]

Shouldn't the source tarball link receive the same treatment?

The sources are a bit different: we need to display the hash, which is long, so we need a way to scroll it horizontally if we want it on the side panel. It's also common to have multiple sources, so a separate section seems sensible.

[sabine]

It is an great improvement (specifically the icons being aligned so neatly, and using better icons goes such a long way).

My only peeve is the removal of the emphasized nature of the documentation link and the repository link. It would be better if these highly important items would still be more emphasized so even new users can find them quickly. I find the documentation link much harder to click now because it is so much smaller.

I suggest returning the documentation to a CTA-button and making the repository link highlighted in orange by default (as previously, except that now it nicely sits together with the other, less important links).