Remove SQLite reserved keywords from sql scripts

[limwa]

uni/uni/lib/controller/local_storage/database/app_exams_database.dart

Line 16 in 90a93fa

CREATE TABLE exams(id TEXT, subject TEXT, begin TEXT, end TEXT,

uni/uni/lib/controller/local_storage/database/app_user_database.dart

Line 12 in 90a93fa

: super('userdata.db', ['CREATE TABLE userdata(key TEXT, value TEXT)']);

BEGIN, END and KEY are all reserved SQLite keywords and should be removed to avoid errors in some SQLite implementations.

SQLite reserved keywords: https://sqlite.org/lang_keywords.html