pools: switch from search[name_matches] to search[name_contains].

The previous commit changed it so that `/pools?search[name_matches]`
does a full-text search. So for example, `search[name_matches]=smiling`
will now match pool names containing any of the words "smiling",
"smile", "smiles", or "smiled".

This commit adds a `/pools?search[name_contains]` param that does what
`name_matches` did before, and switches to it in search forms. So for
example, `search[name_contains]=smiling` will only match pool names
containing the exact substring "smiling".

This change is so that `<field>_matches` works consistently across the
site, and so that it's possible to search pool names by either an exact
substring match, or by a looser natural language match.

This is a minor breaking API change. API users can replace
`/pools?search[name_matches]` with `/pools?search[name_contains]` to get
the same behavior as before. The same applies to /favorite_groups.

app/models/pool_version.rb

@@ -27,7 +27,7 @@ class PoolVersion < ApplicationRecord
       where_array_includes_any(:added_post_ids, [post_id]).or(where_array_includes_any(:removed_post_ids, [post_id]))
     end
 
-    def name_matches(name)
+    def name_contains(name)
       name = normalize_name_for_search(name)
       name = "*#{name}*" unless name =~ /\*/
       where_ilike(:name, name)
@@ -40,8 +40,8 @@ class PoolVersion < ApplicationRecord
         q = q.for_post_id(params[:post_id].to_i)
       end
 
-      if params[:name_matches].present?
-        q = q.name_matches(params[:name_matches])
+      if params[:name_contains].present?
+        q = q.name_contains(params[:name_contains])
       end
 
       if params[:updater_name].present?