Fix meta tags

[aaccioly]

Fixes #36

With this fix users can set their X / Twitter handle for the website, as well as custom metadata per page e.g.:

// Rewrite meta tags for specific pages
// Use the Notion page ID as the key
pageMetadata: {
  'NOTION_PAGE_ID': {
    title: 'My Custom Page Title',
    description: 'My custom page description',
    image: 'https://imagehosting.com/images/page_preview.jpg',
  },
},

It also sets og:url and twitter:url to the slug if one is available, using pageId as a fallback.

With this change I can get my proxied page properly indexed on search engines.

[aaccioly]

@velsa, @DudeThatsErin would you mind having a look? It has neen working very well for me.

[velsa]

Did small code refactor.