Add banner "Upgrade version" for self-hosted when version is older than 6 months

[Albane-dlv]

Detailed Description

Create a banner similar to the one in the mockup below. The banner should appear as soon as the self-hosted version of WEBKNOSSOS has not updated since 6 months. It should be possible to close it, but it should appear again with every new session, until a newer version is used (sessionstore).
Text: You are using an old version of WEBKNOSSOS. Switch to webknossos.org for automatic updates and exclusive features!
Button: Learn more (Links to https://webknossos.org/self-hosted-upgrade)

Context

When users of the self-hosted version of WEBKNOSSOS haven't upgraded since 6 months, this banner should appear, motivating them to switch to webknossos.org. The button "Learn more" will lead them to a landing page where the advantages of wk.org compared to the open-source version are listed.

[philippotto]

Implementation note: could be done similarly to the maintenance banner. /api/buildinfo contains commit date that could be parsed.

[fm3]

Should the user be able to click this away? Do we need to store the info that they clicked it away? I guess the novelUserExperienceInfos block could be used for that.