Vuepress1.0 upgrade

[znicholasbrown]

Thanks for contributing to Prefect!

Please describe your work and make sure your PR:

• adds new tests (if appropriate)
• updates CHANGELOG.md (if appropriate)
• updates docstrings for any new functions or function arguments, including docs/outline.toml for API reference docs (if appropriate)

Note that your PR will not be reviewed unless all three boxes are checked.

What does this PR change?

Resolves #770 by upgrading Vuepress to v1.0.3
Makes the appropriate modifications to migrate to the most recent version, including changes to imports and style overrides.

Why is this PR important?

We were using an unsupported version of Vuepress that didn't allow us flexibility to override components and address issues like #1119

Notes

There are a few places where the style wasn't exactly matched to the live version.
This was for a few reasons:

• sometimes it wasn't worth the effort (and would potentially be breaking)
• it looked better/was clearer in the updated version

Some notable differences are in the sidenav hierarchy styling (the active link is now highlighted in blue when not focused), and the GitHub outbound link is now correctly vertically centered.

[codecov]

Codecov Report

Merging #1408 into master will increase coverage by 0.01%.
The diff coverage is n/a.

[cicdw]

This is awesome, thank you so much! My immediate thought when I saw this was "ooo now we can have a copy icon for code blocks!" and lo and behold, another PR appeared...