Allow rowspan and colspan in markdown table rendering

[jtescher]

Allow rowspan and colspan attributes of forms in READMEs.

Fixes #1863

[rust-highfive]

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[smarnach]

@jtescher Thanks for your contribution! This change looks good to me, but I don't have access to merge and deploy it – some other member of the team will do so soon.

The only thing I'm wondering about is why we don't use ammonia's defaults – they look reasonable to me, and would avoid a bunch of other rendering issues. This is of course unrelated to this PR, and in my opinion we can merge this as it is.

CC @jtgeibel

[sgrif]

I'm actually a bit confused about why percy is failing this, this PR definitely shouldn't have affected download count graphs

[jtgeibel]

I restarted the Frontend job and Percy is happy now, so I'm guessing this was a transient issue on their end.

@smarnach Looking just at the tag_attributes variable, it looks like we additionally allow id and target on a tags. I think id is there to allow hyperlinks to work within the README file. target allows the README to open a link in a new tab. Maybe it would be more reasonable to start with the defaults and then explicitly allow (and document) any such deltas. Would you like to open an issue to track this?

I agree these changes are good to merge.
@bors r+

[bors]

📌 Commit ecb2877 has been approved by jtgeibel

[bors]

⌛ Testing commit ecb2877 with merge af26bb3...

[bors]

☀️ Test successful - checks-travis
Approved by: jtgeibel
Pushing af26bb3 to master...

[smarnach]

I was assuming that the differences in the chart were caused by changes to Google Charts. We use the deprecated jsapi loader that always pulls the latest version of Google Charts, so I wasn't surprised there might be small changes to how the chart looks.

@jtgeibel Yes, we currently do allow a few additional things, e.g. the input element. Since the ammonia API allows amending the defaults with functions like add_tag_attributes(), it would be easy to add these items, but we don't need to put the full list in our code.

I will open an issue for this for further discussion.