Typescript migration part 2 (#261)

Converts everything to typescript, adding `@ts-expect-error`s where
necessary. Also makes some progress converting elements to use
decorators, but we're not fully there yet — I think it'll make sense to
continue to do that in future PRs, rather than doing it all in one go,
just so that we can keep up with the dev branch.

src/pageentry.ts

@@ -18,6 +18,7 @@ class PageEntry extends LitElement {
   textSnippet: string | null = "";
 
   @property({ type: Object })
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any -- TODO fixme
   page: any = null;
 
   @property({ type: String })
@@ -77,7 +78,7 @@ class PageEntry extends LitElement {
         width: 100%;
       }
 
-      /* Overrde Bulma to add the tiniest margin, so the focus indicator isn't obscured */
+      /* Override Bulma to add the tiniest margin, so the focus indicator isn't obscured */
       .columns {
         margin-top: calc(-0.75rem + 2px);
       }