Fix ProjectCacheWorker for plain READMEs

What does this MR do?

The ProjectCacheWorker refreshes cache periodically, but it runs outside Rails context. So include the ActionView helpers so the content_tag method is available.

Are there points in the code the reviewer needs to double check?

I added include ActionView::Context because, I ran into this error:

NoMethodError: undefined method 'output_buffer=' for

See http://stackoverflow.com/a/7562194/89376

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Fixes gitlab-org/gitlab-ce#32540

marked the checklist item Changelog entry added, if necessary as completed

marked the checklist item Documentation created/updated as completed

marked the checklist item API support added as completed

marked the checklist item Conform by the merge request performance guides as completed

marked the checklist item Conform by the style guides as completed

marked the checklist item Branch has no merge conflicts with master (if it does - rebase it please) as completed

marked the checklist item Squashed related commits together as completed

@DouweM WDYT?

added Next Patch Release ~480950 regression labels

changed milestone to %9.2

assigned to @DouweM

changed the description

assigned to @to1ne

approved this merge request

added ~149423 and removed Next Patch Release labels

resolved all discussions

mentioned in commit a3eabcc2

merged

mentioned in issue #32437 (closed)

mentioned in commit d3cc3516

Picked into 9-2-stable, will go into 9.2.0-rc7.