Crate details page: show whether a build failed in the Versions list

[yvrhdn]

Implements second change related to #516.
I was planning to rebase these commits on top of the commits in #543 if that PR gets approved & merged. But this can be merged indepedently.

This pull request changes the Crate Details page to display whether a build failed in the Versions list.

Current design:

Page after these commits:

I performed the changes in two commits, this should make it a bit easier to reason about them. Hopefully...

521b8ae

• I added an extra test case for CrateDetails::versions.
• Most code is the same as 0a56ff7 from PR Crate details page: when a build has failed, display a message that links to last successful build #543, this was necessary to create the test. This code should be removed from the commits when doing a rebase.

da358f8

• In CrateDetails I replaced the field versions with releases. The type changed from a raw String to Release, which holds a version string and a build_status boolean.
• I create a fn versions() which has the same behaviour as the removed versions field.
• I set releases in CrateDetails::new but I took care to change as little code as possible. Hence the introduction of map_to_release. This entire function can be refactored to be more efficient / perform less queries on the database, but that's for a later PR.
• Lastly, I edited crate_details.hbs to use the additional information and show a ⚠️ sign when appropriate.

[jyn514]

In general this looks good! You'll have to rebase over master since #543 got merged. I also left a few minor nits.

[yvrhdn]

I have rebased to the latest commits on master.

Nitpicks have been resolved 👍

And I have simplified the versions display by using a tooltip:

[jyn514]

Looks good to merge once you fix the nit :)

[jyn514]

I'm really looking forward to having this, thanks again for your hard work!