Load release list only when needed

[GuillaumeGomez]

It was something we discussed some time ago. Instead of loading the release list all the time, with this PR it's only loaded when the menu is opened. It should greatly reduce the page size on crates with a lot of releases.

Original issue: #1999

cc @Nemo157 @jsha

[GuillaumeGomez]

I finished porting the code so now JS generates the same thing as jinja used to. The problem now is that some tests relied on the DOM content to check some things, which isn't possible anymore. We could however have the failing tests ported to some "front-end testing"like what #1698 proposes.

[Nemo157]

Rather than doing client-side rendering of json, we could just serve a partial-page of HTML that gets inserted in the DOM, that might be easier to migrate the existing tests to too.

[GuillaumeGomez]

That would indeed make it much simpler to maintain and we could directly re-use the existing template. 👍

[GuillaumeGomez]

I applied suggestions: this path now returns HTML which is directly inserted into the DOM, allowing to greatly simplify the code and also minimize the impact on tests.

[GuillaumeGomez]

@Nemo157: Anything else that needs to be done here?

[GuillaumeGomez]

Applied suggestions.