Initial styling for new room list

This is a work in progress, but covers the coarse areas. This uses all-new classes to better describe what everything is, and to reduce the number of selectors we keep track of.

This is primarily layout for the list and not actually the final structure. For example, some buttons are missing and other areas are not styled correctly - the idea in this commit was to get things roughly in the right place and work on it.

res/css/structures/_LeftPanel2.scss

@@ -0,0 +1,91 @@
+/*
+Copyright 2020 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+$tagPanelWidth: 70px;
+$roomListMinimizedWidth: 50px;
+
+.mx_LeftPanel2 {
+    background-color: $header-panel-bg-color;
+    min-width: 260px;
+    max-width: 50%;
+
+    // Create a row-based flexbox for the TagPanel and the room list
+    display: flex;
+
+    .mx_LeftPanel2_tagPanelContainer {
+        flex-grow: 0;
+        flex-shrink: 0;
+        flex-basis: $tagPanelWidth;
+        height: 100%;
+
+        // Create another flexbox so the TagPanel fills the container
+        display: flex;
+
+        // TagPanel handles its own CSS
+    }
+
+    // Note: The 'room list' in this context is actually everything that isn't the tag
+    // panel, such as the menu options, breadcrumbs, filtering, etc
+    .mx_LeftPanel2_roomListContainer {
+        width: calc(100% - $tagPanelWidth);
+
+        // Create another flexbox (this time a column) for the room list components
+        display: flex;
+        flex-direction: column;
+
+        .mx_LeftPanel2_userHeader {
+            padding: 14px 12px 20px; // 14px top, 12px sides, 20px bottom
+
+            // Create another flexbox column for the rows to stack within
+            display: flex;
+            flex-direction: column;
+
+            // There's 2 rows when breadcrumbs are present: the top bit and the breadcrumbs
+            .mx_LeftPanel2_headerRow {
+                // Create yet another flexbox, this time within the row, to ensure items stay
+                // aligned correctly. This is also a row-based flexbox.
+                display: flex;
+                align-items: center;
+            }
+
+            .mx_LeftPanel2_userAvatarContainer {
+                position: relative; // to make default avatars work
+                margin-right: 8px;
+            }
+
+            .mx_LeftPanel2_userName {
+                font-weight: 600;
+                font-size: $font-15px;
+                line-height: $font-20px;
+            }
+
+            .mx_LeftPanel2_breadcrumbsContainer {
+                // TODO: Improve CSS for breadcrumbs (currently shoved into the view rather than placed)
+                width: 100%;
+                overflow: hidden;
+            }
+        }
+
+        .mx_LeftPanel2_filterContainer {
+            // TODO: Improve CSS for filtering and its input
+        }
+
+        .mx_LeftPanel2_actualRoomListContainer {
+            flex-grow: 1; // fill the available space
+            overflow-y: auto;
+        }
+    }
+}