Make x-pack tags clickable

[nik9000]

This replaces the x-pack tags on the titles of pages that are about
elastic licensed features with clickable links that take you to the
subscriptions page.

This is implemented by replacing the section rendering code in docbook
with customized code that includes the link.

This also replaces the background image of the x-pack tag with one
that looks "more clickable" and has a hover treatment.

Closes #720

[nik9000]

It used to look like:

Now it looks like when you don't hover:

And like this when you do hover:

[nik9000]

I'm leaving this in "draft" state while I write a test for it.

[nik9000]

Ok - this works for sections that have role=xpack but it won't work for chapters with it. I'll see if we have any of those....

[nik9000]

It looks like I will certainly need to do chapters.

[nik9000]

These weren't used.

[nik9000]

Do we have any definitions that are just x-pack? If so I'll need to do something for them.

[lcawl]

I didn't find any in the stack-docs, kibana, or elasticsearch repos

[nik9000]

OK! I've done the chapters too.

[jarpy]

Fancy!

[lcawl]

When I test this with the docbldso (Stack Overview) doc build, it fails to create an icon for this page:
https://github.com/elastic/stack-docs/edit/master/docs/en/stack/security/index.asciidoc (https://www.elastic.co/guide/en/elastic-stack-overview/master/elasticsearch-security.html)
Ditto for all the other pages at that level (that I tested).

[nik9000]

When I test this with the docbldso (Stack Overview) doc build, it fails to create an icon for this page:
https://github.com/elastic/stack-docs/edit/master/docs/en/stack/security/index.asciidoc (https://www.elastic.co/guide/en/elastic-stack-overview/master/elasticsearch-security.html)
Ditto for all the other pages at that level (that I tested).

Ooooh! I'll have a look at that then!

[nik9000]

Ooooh! I'll have a look at that then!

Done! @lcawl, could you have another look around?

[lcawl]

The icons are appearing in the right places now, thanks!
I noticed, however, that they don't take me to a valid URL: file:///subscriptions
Is this something that's a known issue in local builds and will work when it's on the web?

[nik9000]

I noticed, however, that they don't take me to a valid URL: file:///subscriptions

I send you to /subscriptions because I figured that it'd be nicer to make a relative link. I can point them to https://www.elastic.co/subscriptions which'll work locally. I tend to try and avoid these sorts of links because they can lead you from a test environment to a production environment without you knowing it.

[lcawl]

It seems to be encountering some problems in the case of sub-sections too:

The right hand window is the current (desired) formatting (https://www.elastic.co/guide/en/kibana/master/working-remote-clusters.html#managing-cross-cluster-replication) and the left hand window is built with this PR. Note the lack of icon and the reduced size of the title.

The same problem occurs in the Elasticsearch Reference for this X-Pack-tagged section: https://www.elastic.co/guide/en/elasticsearch/reference/master/modules-node.html#modules-node-xpack

[nik9000]

I hit that kind of thing over and over and over again while working on it. Let me see about this instance of it....

[nik9000]

Another thing that I notice is that I add the tag even when the section "above" it also has the flag. This seems wrong but something I can work out.

[nik9000]

@lcawl could you have another scan on this one when you have a chance? I've gotten further I think. And we'll end up with a nice set of tests for when we expect this and when we don't....

[lcawl]

LGTM, thanks! I tested Stack Overview, Kibana Reference, and Elasticsearch Reference.