Fix Encryption scheme

[marcelklehr]

The existing encryption implementation (currently only used for securing your credentials residing in your browser storage), while not strictly broken, was built in a somewhat uneducated fashion. My intention with this PR is to change that.

Here is a list of changes:

1. I've switched to AES in Galois/Counter Mode, which has the benefit of also authenticating the message being encrypted.
2. The module now always uses a random initialization vector and stores it along with the ciphertext.
3. Most importantly the module now makes use of the PBKDF2 key derivation algorithm with 250000 iterations and a salt to make it harder to brute-force the pass phrase.

The changes are transparent to the user: Upon installing and unlocking the new version floccus will automatically decrypt all data with the old implementation and re-encrypt it with the new implementation.

Additionally, this will pave the way for #224