Use buttons instead of links for "mark as read"

See https://github.com/marienfressinaud/FreshRSS/issues/599
Fix bug with shortcut "mark all as read"

See https://github.com/marienfressinaud/FreshRSS/issues/599
Mark many as read must be a POST action

See https://github.com/marienfressinaud/FreshRSS/issues/599