Member avatars without canvas (#9990)

* Strict typechecking fixes for Base/Member/Avatar

Update the core avatar files to pass `--strict --noImplicitAny` typechecks.

Signed-off-by: Clark Fischer <clark.fischer@gmail.com>

* Add tests for Base/Member/Avatar

More thoroughly test the core avatar files. Not necessarily the most thorough,
but an improvement.

Signed-off-by: Clark Fischer <clark.fischer@gmail.com>

* Extract TextAvatar from BaseAvatar

Extracted the fallback/textual avatar into its own component.

Signed-off-by: Clark Fischer <clark.fischer@gmail.com>

* Use standard HTML for non-image avatars

Firefox users with `resistFingerprinting` enabled were seeing random noise
for rooms and users without avatars. There's no real reason to use data
URLs to present flat colors.

This converts non-image avatars to inline blocks with background colors.

See https://github.com/vector-im/element-web/issues/23936

Signed-off-by: Clark Fischer <clark.fischer@gmail.com>

* Have pills use solid backgrounds rather than colored images

Similar to room and member avatars, pills now use colored pseudo-elements
rather than background images.

Signed-off-by: Clark Fischer <clark.fischer@gmail.com>

---------

Signed-off-by: Clark Fischer <clark.fischer@gmail.com>
Co-authored-by: Andy Balaam <andy.balaam@matrix.org>

res/css/views/rooms/_BasicMessageComposer.pcss

@@ -78,7 +78,7 @@ limitations under the License.
                     min-width: $font-16px; /* ensure the avatar is not compressed */
                     height: $font-16px;
                     margin-inline-end: 0.24rem;
-                    background: var(--avatar-background), $background;
+                    background: var(--avatar-background);
                     color: $avatar-initial-color;
                     background-repeat: no-repeat;
                     background-size: $font-16px;