Add a fixed footer to the interface of the app

[mahendrark]

Fix #Issue

Changes proposed in this pull request:

• Add a fixed footer

The following steps were realized, as well (if applies):

• Apply black (black . --exclude docs/)
• Update the CHANGELOG.md

The following optional steps were realized:

• Use in-line comments to explain your code
• Write docstrings to your code
• For new functionalities: explain in readthedocs
• Write test(s) for your new patch of code

_{For more information on how to contribute check the [CONTRIBUTING.md](https://github.com/rl
-institut/open_plan/blob/dev/CONTRIBUTING.md).}

[Bachibouzouk]

For me the footer is not fixed to the bottom of the page. I think it is fine if the footer does not eat up space on the screen, but is available upon scrolling