Add database spec document and establish lesson ids

doc/db_spec.md

@@ -0,0 +1,50 @@
+# Database specification
+
+## Lesson IDs
+
+`lesson_id` and `g*` equivalents of lessons.
+
+- Academic Writing
+    - 'aw'
+- Biology
+    - 'bi'
+- Business Management
+    - 'bm'
+- Chemistry
+    - 'ch'
+- Computer Science
+    - 'cs'
+- Economics
+    - 'ec'
+- English A
+    - 'ena'
+- English B
+    - 'enb'
+- French
+    - 'fr'
+- Geography
+    - 'ge'
+- German
+    - 'de'
+- Guidance
+    - 'gu'
+- Mathematical Studies
+    - 'mst'
+- Mathematics HL
+    - 'mhl'
+- Mathematics SL
+    - 'msl'
+- Polish A
+    - 'pl'
+- Psychology
+    - 'ps'
+- Self-Taught Language A
+    - 'st'
+- Spanish
+    - 'es'
+- Theory of Knowledge
+    - 'tok'
+- Turkish A
+    - 'tr'
+- Visual Arts
+    - 'va'