Rework project structure

[Arnei]

The current branch structure of the project is ill-fit to work with Opencast. To make it easier to keep track of changes and release to Opencast, we should adopt their structure. This means

• Renaming our admin-ui-picard branch (master) to develop
• Establishing version-number.x branches, e.g. 14.x

[ziegenberg]

Renaming just happened to main.

[KatrinIhler]

Just discussed in the Dev meeting and we all agreed that the Admin Interface should adopt the branch structure of the main repository so we can get bug fixes into the legacy releases without being blocked by breaking changes.

[mtneug]

How would releases be tagged?

[KatrinIhler]

See opencast/opencast-editor#1411 for that discussion.