Add YAML-metadata for notes and change the document title accordingly (#310)

* Added yaml-frontmatter extracting and error handling
* add tests
* changed document-title, so the editor can change the title to the title of the yaml metadata. closes #303
* extracted first line parsing in a core rule of markdown-it
document title will now be determined like this:
1. yaml metadata title
2. opengraph title
3. first level one heading
4. 'Untitled'
* added documentTitle e2e test

Co-authored-by: Erik Michelson <github@erik.michelson.eu>
Co-authored-by: Philip Molares <philip@mauricedoepke.de>
Co-authored-by: Tilman Vatteroth <tilman.vatteroth@tu-dortmund.de>
Co-authored-by: mrdrogdrog <mr.drogdrog@gmail.com>

cypress/support/config.ts

@@ -3,6 +3,11 @@ export const banner = {
   timestamp: '2020-05-22T20:46:08.962Z'
 }
 
+export const branding = {
+  name: 'ACME Corp',
+  logo: 'http://localhost:3000/acme.png'
+}
+
 beforeEach(() => {
   cy.server()
   cy.route({
@@ -22,10 +27,7 @@ beforeEach(() => {
         email: true,
         openid: true
       },
-      branding: {
-        name: 'ACME Corp',
-        logo: 'http://localhost:3000/acme.png'
-      },
+      branding: branding,
       banner: banner,
       customAuthNames: {
         ldap: 'FooBar',