Vanilla Porter 2.6 RC1

Hi all,

No one's done a release of Vanilla Porter since I left Vanilla nearly 2 years ago, so I've updated my fork with the additional PRs on the main repo and built a potential release. I no longer have repo access, but I do have admin access here to update the plugin.

Would you please try this and let me know if it works? Any test data you have is great. There is no test coverage on this damn thing (my bad) and so all I can do without test data is confirm the new build doesn't have any build (fatal) errors in it.

I've versioned this as 2.6. Uncredited updates were generally by Olivier, a Vanilla employee.

Release notes:

1. Q2A: NEW package supported (thanks Eduardo Casarero / edux!)
2. MVC Forum: NEW package supported
3. Improve utf8mb4 support (thanks @Bleistivt !)

Package updates:

1. AnswerHub: Add Q&A, attachments, and tags support.
2. myBB: Add support for attachments (thanks Javier-Rotelli!)
3. phpBB3: Add "viewed" data to imported discussions.
4. vBulletin: Remove stripping of the ATTACH tag. (thanks @MrCaspan !)
5. vBulletin: Fix accidentally importing deleted PMs.
6. vBulletin: Add reactions and tags.
7. Xenforo: Improve performance when migrating large sites.
8. Xenforo: Fix database prefix issue.