Use first achievement as a reference for the average points per week

[Zekromaster]

As discussed on the Discord, I altered the calculation of the average points per week so that instead of using the join date as a starting date, they use the date of the first achievement on the profile.

[wescopeland]

@Zekromaster I like to be the one who marks my comments as resolved, otherwise I lose track of what I need to review when reapproaching a PR. Thanks for your understanding on this. For the time being, I've "unresolved" the given comments.

[Zekromaster]

@Zekromaster I like to be the one who marks my comments as resolved, otherwise I lose track of what I need to review when reapproaching a PR. Thanks for your understanding on this. For the time being, I've "unresolved" the given comments.

No problem, sorry for the confusion!

[Zekromaster]

I'm assuming all commits will be squashed before merge, so I'm just pushing another "apply code review" commit. I manually reverted what had to be reverted and tested squashing locally, now UserProfileMeta.php is the only file that would get touched by squash-merging.