Make bookmarks items' list fixed

[cezaraugusto]

PR #8532 addressed some issues regarding bookmarks but fixed header for item's list was left. That's because list uses SortableTable, and per spec tables aren't supposed to have overflow.

Screenshot credits of @jonathansampson:

Ideally, items' header (right-hand side) should be fixed.

In order to fix that, SortableTable should be either fully refactored to make use of CSS grid or flagged at some point that other element than <table> (and its children) should be used.

Labelled as backlog feel free to remove if needed

[cezaraugusto]

Removing good-first-bug since refactoring SortableTable can be tricky. Feel free to ping me anyone willing to take it.

[luixxiul]

fyi there is an upstream bug: https://bugs.chromium.org/p/chromium/issues/detail?id=702927 "position: sticky does not work on or ". Since sortableTable will be deprecated with another component, it'll be not an issue for us.

[rebron]

Removing from backlog. Not an issue in brave-core.