This byline includes both repository and maintainers where available in metadata. This replaces the use of maintainers in footer.

See commit message of nextstrain/nextstrain.org@fc4aee3 for rationale

Auspice (2.*) allows the `getAvailable` API request to define `buildUrl`s for datasets. These were displayed in the sidebar, above the choose-dataset dropdowns. 

This commit removes the sidebar display of this, as it's been super-seeded by the byline.

The commit also sets the `metadata.buildUrl` to be that defined in the `getAvailable` API response _if_ it has not been set via the dataset JSON, as the latter is to be considered the "canonical" way of defining this value.

Refactors the byline into its own React Component. In general, I think this helps reduce interpretability of the code. No changes in UI. Minimal changes to implementation, related to making this a functional component which are simpler than react classes (and react is generally moving away from classes).

Feels much cleaner to me, but this is highly subjective!

The "should i render" boolean approach is replaced with returning a react component when it's determined that something should be rendered, else returning `null` as a fallthrough at the end of the function (null is better than `<span/>`). 

The `renderLink` function is replaced by a component, which gets passed props rather than an object.

Looks like lots of changes but not that many really!

The originally intention was to have the overall weight (including size, color and font weight) of byline to be about halfway between title and info. I see that the weight 700 wasn't working. This looks a little better to my eye while still incorporating direction implemented by @jameshadfield.