Initial stab at a security tab showing advisories

[djc]

Here's my first take at implementing the security tab as proposed in RFC 3872. Resolves #12507.

This is intentionally light on details to get an initial review on my first baby Ember.js steps. Not sure how much we need to add before this gets merged.

Keeping as draft while the RFC has not been merged.

[djc]

I've implemented Markdown formatting with GitHub-flavored Markdown support for the advisory details.

Seems like a decent MVP from which we can iterate further?

[Turbo87]

looks like a good start! 👍

one more suggestion that did not fit into the diff: we should add tests for this new feature to https://github.com/rust-lang/crates.io/tree/main/e2e/acceptance. at least one test for displaying 2+ advisories, one for the empty state, and one for the rustsec server returning e.g. a 500 error. we use msw for network mocking, which should make it relatively straight-forward to implement. it's probably sufficient to implement the msw request handlers in the tests instead of creating a generic handler in the crates-io-msw package.