Update PHP coding standards to avoid calling static methods with self

[GuySartorelli]

We had a bug which it turned out was caused by an unexpected interaction between self and late static binding (see this PR).
This was raised in php-src where the response was that it was working as intended.

Going forward we should avoid calling static methods from self to avoid this class of bug.

Acceptance Criteria

• The PHP coding standards are updated to explicitly disallow calling static methods with self (e.g. self::get())
• The docs mention the rationale, i.e. we're avoiding a specific class of bug that is introduced when using late static binding downstream from self
• static is fine, because it explicitly uses late static binding anyway

TO DECIDE

• Are there any scenarios where self is allowed? e.g. is self okay in a type hint, and is self::class okay? Or is is better to just avoid self altogether (e.g. use the explicit class name, or __CLASS__ as a replacement of self::class)?

[maxime-rainville]

Is there an actual issue that got cause by this? e.g. Did we have a scenario where we called self::get() in Link wanting to query all Links and ended up querying only FileLink.

[GuySartorelli]

Is there an actual issue that got cause by this?

Yes. I'll link you the slack thread.