#5689823 Partner Portal: Add an empty view for the license list

Context:

• Project thread: pbtFPC-Um-p2
• i4 designs: pbtFPC-103-p2

Changes proposed in this Pull Request

• Move the license list header into its own component
• Introduce a new component for the UI of the empty views in the license list.

Testing instructions

• If you do not have a partner key, please contact Infinity for one or you will be unable to test this PR.
• Check out this PR locally, then run* yarn && yarn start-jetpack-cloud.
• Visit http://jetpack.cloud.localhost:3000/partner-portal
• Filter the list by one of the status: All Active, Unassigned, Assigned, and Revoked
• If it doesn’t have licenses after a certain filter, it should show the title (eg. “No Assigned Licenses”) and a button to issue a new license.
• If it doesn’t have unassigned licenses and there are assigned licenses, and you are under the “Unassigned” filter, show another line with the following message: “Every license you own is currently attached to a site.”