1. In many cases, the `flex-list` has previous and next `gt-hidden`
siblings, so relax the CSS selector to remove all ".segument .flex-list"
paddings.
2. Make the "Add key" button can toggle
3. Move help message into the related segment(panel). Otherwise users
would misread the message, eg: the SSH help seemed for GPG because they
are so near
4. Move modal element into the segment element, otherwise it affects the
layout
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