Database is inconsistent about QString and QByteArray, causing a high risk of bugs

[TobiasFella]

When storing keys in the database, libQuotient sometimes uses QByteArray and sometimes QString, depending on the type the variable has in the C++ code. When trying to load a key that was stored as a byte array as a string (or vice-versa), the result is empty, leading to unexpected and very difficult to figure out bugs.

A future database migration should

• Migrate all existing keys from byte arrays to strings
• Change all existing code to read and write strings
• Ideally also adjust API to make sure that we keep keys as strings as much as possible