css: refactor CSS to use Tailwind-style utility classes.

Refactor CSS to use standard Tailwind-style utility classes instead of
ad-hoc rules. This eliminates a lot of single-purpose rules for specific
UI elements and standardizes margins to be more consistent throughout
the site.

Utility classes are defined manually on an as-needed basis instead of
importing Tailwind as a whole. Naming conventions mostly follow
Tailwind's conventions, otherwise they follow Bootstrap.

* https://tailwindcss.com/docs/
* https://getbootstrap.com/docs/5.0/utilities/spacing/

app/views/saved_searches/index.html.erb

@@ -36,7 +36,7 @@
       <% t.column "Created" do |ss| %>
         <%= time_ago_in_words_tagged ss.created_at %>
       <% end %>
-      <% t.column th: {class: "links"}, td: {class: "links"} do |ss| %>
+      <% t.column column: "control" do |ss| %>
         <%= link_to "edit", edit_saved_search_path(ss) %>
           | <%= link_to "delete", saved_search_path(ss), :method => :delete, :remote => true %>
       <% end %>