Move primer to yarn

[dometto]

Take 2. :)

It tuns out the 'product' component is necessary to style things like tables in the markdown body. However, this does not work neatly unless we import the primer components after importing our own template.scss. In turn, this caused primer to override some of our own styling from template.scss. I have addressed this by identifying which parts of template.scss are superfluous, which need to be included before primer, and which need to be included after primer.

• Move our own .markdown_body styling to a separate file
• The spinner with 'When was this page last modified?' is no longer there
• Rebuild assets

[dometto]

@benjaminwil @bartkamphorst I believe this solves all the styling issues @bartkamphorst discovered after having (prematurely) merged #1914. The problem was that some of our own styling from template.scss was being overridden by primer. Annoyingly, however, importing template.scss after primer also didn't work (presumably because some primer definitions were being used in template). So in this PR, I have:

1. Radically cut some styling from template.scss that it looks like primer is already providing. I have tested as best as I could, but it would be great if you could give this branch a spin and see if you see anything strange.
2. Moved some of the styling from template.scss that we don't want primer to override into app.scss, after primer is included.

If this approach seems sane, I'll move the new styling in app.scss to a separate new file (something like wiki_content.scss).

[bartkamphorst]

Overall it seems to look okay. Two things I noticed straight away:

1. The styling of the tables no longer has the rounded corners from Add rounded corners to tables. Resolves #1693. #1881 .
   

EDIT: This seems to be straightforwardly caused by the table styling being removed from template.scss.erb.

2. The spinner with 'When was this page last modified?' is no longer there.

[dometto]

This seems to be straightforwardly caused by the table styling being removed from template.scss.erb.

Thanks for catching! Re-added that styling.

The spinner with 'When was this page last modified?' is no longer there.

Again thanks for catching. Not sure yet why.

I've added the spinner issue to the todo for this PR. Apart from that issue, I think this is good to go. We may of course still discover slight differences in the styling in the future, but if they need to be addressed at all, it should be straightforward to fix them by looking at the changes in this PR.

[bartkamphorst]

The problem with the spinner can be straightforwardly resolved by adding back the //= require spinner directive in app.scss. 😄

(Of course, that only restores the dotted-spinner but does not yet replace it with Primer's spinner component (see #1925).)

[dometto]

Ok, I'm merging this. This was more complicated than I hoped, and as I said there may be tiny style issues that I've missed. But I think it should be much easier to fix such issues now (also after #1915). Thanks for the feedback @benjaminwil and @bartkamphorst!