[statistics] fix: add missing last period to custom date ranges

[davwheat]

Fixes a bug introduced with #3622.

Changes proposed in this pull request:

• adds support for "last period" for custom date ranges
• fix incorrect offset for previous date ranges

Reviewers should focus on:

• N/A

Screenshot

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.