Display the core version in the sidebar

[zanieb]

☁️ This feature cannot be merged until core_version is in prod. See tracking

PR Checklist:

• add a short description of what's changed to the top of the CHANGELOG.md
• add/update tests (or don't, for reasons explained below)

Describe this PR

Adds the core version from PrefectHQ/server#126 to the state and displays it in the sidebar under the Prefect logo

Notes:

• The core version will only display for Server because it is not meaningful from Cloud
• For sanctioned versions, the +... commits ahead of master cruft should not be there but a release of Server needs to be cut for me to show it a screenshot

[zanieb]

Closes PrefectHQ/server#120

[zhen0]

LGTM @madkinsz - @znicholasbrown anything to add or shall we merge?

[zanieb]

Nothing more from me!

[znicholasbrown]

We'll wait until the next Core release to merge to make sure this doesn't break Prefect-hosted but Server-backed setups.

[znicholasbrown]

This should be good to go with the release of 0.13.16