Issue: No Upgrade Pending Message if Platform Was Upgraded

If the forum platform is upgraded past a major version threshold, then new wiki features may become active for the new platform version. However, the wiki database may require changes to accommodate that. Without those changes, database errors may occur.

Some obvious times this may occur:
- XenForo is upgraded past 1.3 (table xf_bb_code is added)
- vBulletin 3 is upgraded past 4.0 (table contenttype is added)

The wiki should become unusable if the platform version would activate features that will cause database errors, instead showing a message that the wiki must be upgraded.

In order to handle this, a second version string, vw_platform_string must be created that gets updated whenever VaultWiki is upgraded, where VaultWiki marks the platform version at the last upgrade. This will allow VaultWiki to calculate whether the current platform version does not match the last platform version. If not, and if a known major threshold occurs between them, then the wiki should be automatically replaced with an upgrade pending message.