standardize resource_link shortcode's output

[HussainTaj-arbisoft]

Description/Context

At the time of this writing, our resource_link shortcode can produce two types of output.

• HTML for external-resources
• Markdown for other types of links

Studio uses this shortcode with {{% tags i.e {{% resource_link ... %}}. {{% syntax processes markdown content while the alternative {{< syntax does not.

The shortcode should return one kind of output so that the use of the shortcode follows a consistent syntax. Mixing the output types has the potential to cause unexpected rendering problems.

For information on the issues we have faced, you may read #1371 (comment). And for more details about the issue itself, you may read this excellent explanation by @gumaerc.

Plan/Design

The approach we follow relies on the future of External Resources. If their UI flow changes, we might not need to change resource_link. We can use the shortcode external_resource_link instead. However, if the UI flow remains the same, we will need to update the resource_link shortcode to return only HTML and update all of its usage.

(Originally proposed in #1371 (comment))

The latter will require the following steps:

1. Update resource_link to return only HTML.
2. Update Studio's CKEditor to support {{< resource_link ... >}} syntax.
3. Update the usage inside existing content and publish the updates.