Extract "issues_list.scss" into its own under page_bundles.

Move .issue_list into "page_bundles/issues_list.scss"
from "pages/issues.scss"

Merge "page_bundles/issues.css" which contained stylings
,.user-can-drag and .is-ghost, only relevant to issues list
into "page_bundles/issues_list.scss".
(boards and epic duplicate .user-can-drag and .is-ghost
in their own scss).

Use gray-10 over gray-light as background color
for displaying issue list when in manual ordering mode.