feat: Change multiplier to 7/8 when scaling down avatars

The resolution-limits for avatar-images are currently 512x512 or 256x256.
Reducing the resolution further, to 2/3 each step, can reduce the quality
much more than is necessary to fit within the file-size-limits,
which are currently 60 kB or 20 kB.

An image made entirely of noise (which results in unusually
large file-sizes), encoded with jpeg-quality 75,
and 4:2:2-colour-subsampling (the format currently used
for encoding images), can be below 60 kB at 227x227.
For the lower file-size-limit of 20 kB,
such images can be too large at 170x170, but fit at 149x149.
More normal images will have a lower file-size at the same resolution.

Before this change, the target-resolutions for resampling were:
512x512 ->                       341x341 ->                       227x227.

And for the lower file-size-limit:
256x256 ->                       170x170 ->                       113x113.

After this change, the target-resolutions for resampling will be:
512x512 -> 448x448 -> 392x392 -> 343x343 -> 300x300 -> 262x262 -> 229x229.

And for the lower file-size-limit, those will be:
256x256 -> 224x224 -> 196x196 -> 171x171 -> 149x149 -> 130x130 -> 113x113.

This does add 2 steps between the previous target-resolutions,
while still reaching target-resolutions close to the previous ones,
to reduce situations in which the quality will be lower than before.

src/blob.rs

@@ -487,7 +487,7 @@ impl<'a> BlobObject<'a> {
                             ));
                         }
 
-                        target_wh = target_wh * 2 / 3;
+                        target_wh = target_wh * 7 / 8;
                     } else {
                         info!(
                             context,