Improve style rules for thread summary (#8868)

* Use mixin ThreadSummaryIcon

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

* Tidy mx_ThreadSummary

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

* Move style blocks from _EventTile.scss to _ThreadSummary.scss

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

* Merge mx_ThreadSummaryIcon::before

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

* From threads amount to replies amount

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

* Remove obsolete declaration and class names

mixin ThreadSummaryIcon has "background-color: $secondary-content !important"

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

* Move mx_ThreadPanel_replies::before from _ThreadSummary to _EventTile.scss

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

* Rename mx_ThreadSummaryIcon to mx_ThreadSummary_icon

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

* Use variables and remove obsolete one

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

* Merge style rules, renaming a variable

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

* Include mx_MessagePanel_narrow in mx_ThreadSummary

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

* Remove a redundant declaration

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

* Use a variable

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

* Include mx_ThreadSummary_sender and mx_ThreadSummary_content in mx_ThreadSummary

Expected according to tests

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

* Remove a variable used only once

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

* Ensure the same line-height is applied

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

* Remove !important

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

src/components/views/rooms/ThreadSummary.tsx

@@ -58,7 +58,7 @@ const ThreadSummary = ({ mxEvent, thread }: IProps) => {
             }}
             aria-label={_t("Open thread")}
         >
-            <span className="mx_ThreadSummary_ThreadsAmount">
+            <span className="mx_ThreadSummary_replies_amount">
                 { countSection }
             </span>
             <ThreadMessagePreview thread={thread} showDisplayname={!roomContext.narrow} />