[FIX] web_editor: restore "load more" image button as button style

With commit [1] the "load more" button of the media dialog went from a
btn-primary style to a btn-secondary style, which is displayed a link.
It was done in an Odoo global effort of making each and every screen
having only one (sometimes two at most) primary action to help the UX,
as explained at [2].
But in this case, it seems to more misses with the UX than helping the
user as that button really becomes invisible without it. We already were
afraid of that but still merge it, see [2] again.
Indeed, in secondary style this button is barely visible due to the
unsplash and undraw image results which have an "author" text like on
top of the image which looks like the "load more" style.

This commit is using the `btn-odoo` class to show it as a button and not
a link to avoid people not seeing it.
`btn-odoo` is chosen instead of `btn-primary` as this is how "secondary
important actions" are designed now in Odoo.

Note that in community, it looks like a btn-primary but that's not
related to this PR.

[1]: https://github.com/odoo/odoo/commit/ff0b2d441252560a076a3609d81c369f0ce42d19
[2]: https://github.com/odoo/odoo/pull/98706#issuecomment-1236804203

closes odoo/odoo#105193

X-original-commit: f204d3a6ed5b5de08c7921a0bcd5188d78b0cf8b
Signed-off-by: Romain Derie (rde) <rde@odoo.com>

addons/web_editor/static/src/components/media_dialog/file_selector.xml

@@ -58,7 +58,7 @@
             multiSelect="props.multiSelect"/>
         <t t-call="{{ constructor.attachmentsListTemplate }}"/>
         <div name="load_more_attachments" class="mt-4 text-center mx-auto o_we_load_more">
-            <button t-if="canLoadMore" class="btn btn-secondary o_load_more" type="button" t-on-click="loadMore">Load more...</button>
+            <button t-if="canLoadMore" class="btn btn-odoo o_load_more" type="button" t-on-click="loadMore">Load more...</button>
             <div t-if="hasContent and !canLoadMore" class="mt-4 o_load_done_msg">
                 <span><i t-esc="allLoadedText"/></span>
             </div>