[2.1] Upgrader - Fix issues with json conversion

[sbulen]

Partial for #9259

The json conversion step had some issues.

• Due to a typo, safe_unserialize() string fixes were never applied.
• json_encode can ONLY work on UTF8. Nothing else. Anytime a non-utf8 character was passed, the json_encode would fail & return false.
• The result was not checked, & "false" was passed to updates, which, for example, might tell updateSettings() to decrement the value of the setting. MariaDB would crash rather than comply; MySQL would change affected settings to -1, e.g., memberlist_cache.
• The json conversion occurs before the UTF8 conversion... So this would happen even in latin1 databases (umlauts & accents, etc.).

The net result is that many strings were either not converted or blanked out. Settings might be affected, e.g,. memberlist_cache. Most of the issues were with various logs, e.g., log_actions. qanda. Places where user-entered text ended up in a serialized string.

I use a simple "cheezy 2.1 encoding" technique to preserve the non-utf8 strings across json_encode. (Trademark pending.) Got a better idea, let me know... But it works. The UTF8 conversion, which comes later, works as desired & the content is properly migrated to UTF8.

This one is ready to go. If approved, I'll work on the 3.0 version.

[sbulen]

Examples of the types of issues you might see...

In smf_settings:

In smf_log_actions:

[sbulen]

If json_encode DID find proper unicode, its default was to escape it, which was bad for SMF:

[sbulen]

Found an issue, need to do some retests...