Use consistent spelling for request.referer

What does this MR do?

Uses consistent spelling for request.referer.

• 'referer', although gramatically incorrect, is the spelling in the HTTP spec, as well as the actual (non-aliased) method name on the request object.
• It is also spelled as 'referer' in other existing places in the app.

See also:

• ba181565
• !52385 (comment 591568296)

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
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• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.