Improve code comment review on mobile (#21461)

- Fix placement of avatar image, this was not placed in the
`comment-header-left` and add CSS to cover the limiting of width+height
of avatar for code-review comment on "Files changed" page. This fixes
the big noticeable avatar issue.
- Apply `margin-bottom` to the "next" button, so it's consistent with
the "previous" button.
- Make sure the "next"/"previous" start at `flex-start` on mobile and
not off-screen at `flex-end`. As well force them to have `flex: 1` so
they won't overflow on x-asis. This also requires the `width: 100%` for
the `.ui.buttons` div.
- Resolves #20074


### Before
<details><img width="512"
src="https://user-images.githubusercontent.com/25481501/195952930-09560cad-419f-43a3-a8a4-a4166c117994.jpg"></details>

### After
<details><img width="512"
src="https://user-images.githubusercontent.com/25481501/197340081-0365dfa8-4344-46b4-8702-a40c778c073f.jpg"></details>

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>

web_src/less/helpers.less

@@ -175,4 +175,5 @@
 @media @mediaSm {
   .db-small { display: block !important; }
   .w-100-small { width: 100% !important; }
+  .js-small { justify-content: flex-start !important; }
 }