refactor package urls, routes, and handlers

[sabine]

This patch

• renames package urls and handlers to better reflect their purpose and be more consistently named
• introduces dedicated url and handler for package files (README, CHANGELOG, LICENSE)
• moves all non-handler package-related functions into a module PackageHelpers to prevent confusion with handlers
• extends Ocamlorg_frontend.Package by homepages and source

Resolves #995.

There's more potential for tidying up / clarifying that this patch does not include, e.g.

1. sidebar state of package_documentation handler
2. extend voodoo to inspect for README, CHANGELOG, LICENSE and add info about these files to package.json
3. make voodoo emit documentation_status as part of package.json
4. after 2+3) make Ocamlorg_frontend.Package pick up this info from package.json

[cuihtlauac]

I've uploaded a branch with some changes. Nothing significant, just helped me review. Feel free to pick and choose.

https://github.com/ocaml/ocaml.org/tree/cuihtlauac-999

It may have been possible to split this in several PR.

In url.ml I'm not so convinced by the names:

package_redirect
package_docs_redirect 

To me, redirection is an implementation option which is not relevant to URL string processing and should not be exposed. Maybe package_overview_without_version and package_documentation_without_version would be better. I'm not sure.

Overall, it goes in the right direction, this is what matters.

[sabine]

In url.ml I'm not so convinced by the names:
package_redirect
package_docs_redirect
To me, redirection is an implementation option which is not relevant to URL string processing and should not be exposed. Maybe package_overview_without_version and package_documentation_without_version would be better. I'm not sure.

I'm pretty sure I don't want to name it in such a way that someone will be tempted to use that URL in a template. I considered moving it to the redirection.ml file, as it's just a redirect. However, there were no "dynamic redirects" happening in that file and that discouraged me. Now that I think about it again, I think I should move the redirects to redirection.ml.

[cuihtlauac]

Now that I think about it again, I think I should move the redirects to redirection.ml.

I had the same thought. Let's do that and remove them from Url