Fix the /pools/gallery page layout being broken by 8841de68a.
This required refactoring the PostGalleryComponent to take a set of
PostPreviewComponents instead of a set of Posts.
The upshot is that it's technically possible to have adjustable
thumbnail sizes on the pool gallery page now (although this is not yet
exposed in the UI).
Factor out thumbnail galleries into a PostGallery component.
This changes the html structure so that post galleries on all pages are
always wrapped in a `.posts-container` class. This fixes an issue with
thumbnails on the pool show page not being aligned correctly on mobile,
like they are on the post index page. This also affected thumbnail
galleries on other pages, like wiki pages and user profiles.
* Refactor the paginator into a ViewComponent.
* Fix inconsistent spacing between paginator items.
* Fix a bug where the sequential paginator generated the wrong next /
previous page links in the <link rel="{next|prev}"> tags in the <head>.
* Always include the final page as a hidden html element, so that it can
be unhidden with custom CSS.
* Make it easier to change the pagination window.
Add <link rel="prev"> and <link rel="next"> tags to the forum topic show
page, the pool show page, and the favgroup show page.
This is kind of useless now since Google hasn't used these tags for SEO
for years ([1]), and userscripts can use `#paginator-next` and
`#paginator-prev` to find the next/prev pages, but whatever.
[1]: https://yoast.com/google-doesnt-use-rel-prev-next-for-pagination
