Merges #164 and adds the --engine webcrypto option to use the WebCrypto implementation. Adding this doesn't break existing remember-me/auto-decrypt share links and using the flag raises the count of PBKDF2 iterations to 600k, as was the target in #159.
To use it the password_template needs to be updated, so we nudge users towards that engine only if we detect the template supports it.
Supporting backward compatibility and various types of nudges adds significant complexity to the code and number of cases to have in mind when testing, so this is one of the last feature on StatiCrypt 2.x and I'll mostly work on the v3 from now on.