Improve style rules of threads list (#8706)

* Remove obsolete declarations - border-radius of EventTile_line

The border-radius is specified with .mx_EventTile[data-shape="ThreadsList"]

Signed-off-by: Suguru Hirahara <luixxiul@users.noreply.github.com>

* Move declarations for ThreadSummary on Threadslist from _ThreadsPanel.scss to _EventTile.scss

To .mx_EventTile[data-shape=ThreadsList]

Signed-off-by: Suguru Hirahara <luixxiul@users.noreply.github.com>

* mx_ThreadPanel_replies block is used only on threads list after all

Signed-off-by: Suguru Hirahara <luixxiul@users.noreply.github.com>

* &::after and &::before

Signed-off-by: Suguru Hirahara <luixxiul@users.noreply.github.com>

* Set shorthand property and use variables

- Shorthand for inset property of border between threads
- Use variables

Signed-off-by: Suguru Hirahara <luixxiul@users.noreply.github.com>

* Change 'ThreadPanel_ThreadsAmount' class name to 'ThreadPanel_replies_amount'

Signed-off-by: Suguru Hirahara <luixxiul@users.noreply.github.com>

* specific to prevent regressions

Signed-off-by: Suguru Hirahara <luixxiul@users.noreply.github.com>

src/components/views/rooms/EventTile.tsx

@@ -518,7 +518,7 @@ export class UnwrappedEventTile extends React.Component<IProps, IState> {
         }
 
         return <div className="mx_ThreadPanel_replies">
-            <span className="mx_ThreadPanel_ThreadsAmount">
+            <span className="mx_ThreadPanel_replies_amount">
                 { this.state.thread.length }
             </span>
             <ThreadMessagePreview thread={this.state.thread} />