feat: Backlinks (#979)

* feat: backlinks

* feat: add backlinkDocumentId to documents.list

* chore: refactor
fix: create and delete backlink handling

* fix: guard against self links

* feat: basic frontend
fix: race condition

* styling

* test: fix parse ids

* self review

* linting

* feat: Improved link styling

* fix: Increase clickable area at bottom of doc / between references

* perf: global styles are SLOW

shared/utils/parseDocumentIds.test.js

@@ -0,0 +1,20 @@
+/* eslint-disable flowtype/require-valid-file-annotation */
+import parseDocumentIds from './parseDocumentIds';
+
+it('should return an array of document ids', () => {
+  expect(parseDocumentIds(`# Header`).length).toBe(0);
+  expect(
+    parseDocumentIds(`# Header
+  
+  [title](/doc/test-456733)
+  `)[0]
+  ).toBe('test-456733');
+});
+
+it('should not return non document links', () => {
+  expect(parseDocumentIds(`[title](http://www.google.com)`).length).toBe(0);
+});
+
+it('should not return non document relative links', () => {
+  expect(parseDocumentIds(`[title](/developers)`).length).toBe(0);
+});