Fix 'Interactive sequence view' scrolling to always show menu

[nataled]

If a term has many children to view in the alignment view, it becomes necessary to scroll to see all. As one does so, the menu (containing the summary and the link to the zoomed-in view) disappears. This is not desirable. Two possible fixes:

1. Change the scroll to only affect the sequence representations, leaving the menu area in place.
2. Allow the user to expand the viewer by drag and drop.

Both are useful, but if only one is implemented, solution (1) is preferred.

Example term: https://proconsortium.org/app/entry/PR%3A000036285/

[nataled]

I also just noticed that the left-right scroll bar located at the bottom of the alignment will also need to stay in view.

[Julie-Cowart]

better example (that actually loads is Smad2 https://proconsortium.org/app_test/entry/PR:000000364/)

[Julie-Cowart]

Sticky header is implemented.

Scroll bars at the bottom always being in view is going to be tricky since that's an embedded page (linked to https://proconsortium.org/app_test/visual/msa/view/entry/PR:000000364/) that has contents longer than the viewport so the browser just adds the scroll bars.

[nataled]

Not fully resolved, but can be pushed to production for now. Will look into future improvements (such as always-in-view numbering and bottom scroll bar).