change cipher to aes-128-gcm

[sieveLau]

What this does:

1. split the javascript used for decryption into a seperate file
2. change cipher from aes-cbc to aes-gcm

About 1, pros and cons are described in #20

Reasons for 2:

• see #18

Dependencies changed:

• CryptoJS doesn't support aes-gcm, I use forge to fulfil the requirement
• pycrypto doesn't support aes-gcm, I use pycryptodome to fulfil that

[sieveLau]

If we accept to use aes-gcm to encrypt the content, the password hashing part would be the next part to be strengthened. After all, md5 is too fast for password hashing, but we need a slow one.

On the other hand, if the encryptor is fed with a lot of htmls which require encryption, the hashing algorithm can't be slow.

[zerofancy]

This can also fix #24 . https://stackoverflow.com/questions/70705404/systemerror-py-ssize-t-clean-macro-must-be-defined-for-formats

So are there any update on this?