Move the content of the subheader into a footer

[megoth]

The grey line that shows the owner of the current Pod (that is shown when you're visiting another Pod) can be a bit obtrusive. We suggest that we move it into a footer.

Additionally, we suggest to make this footer "sticky", as described at https://css-tricks.com/couple-takes-sticky-footer/ .

[megoth]

Tim also suggest us adding a link to https://solidproject.org/ and to link to the root of the Pod in the text "the Pod".

[megoth]

I propose to make the footer as simple as possible, as shown in the example below:

[megoth]

This was done in SolidOS/mashlib#49