feat: make video embeddings optional. (alshedivat#2337)

As discussed in alshedivat#1181, I suggest to make embedding videos an optional
feature.

This behavior aligns well with recently merged PR alshedivat#2312.

Open questions:

1. I added a youtube link to `papers.bib`. Is this link okay?
2. I set `enable_video_embedding: false` as the default. I argue that
privacy settings should be the default. Also, the current implementation
of `video.liquid` only works for some very specific video URLs. For
example, to embed youtube, specialized links must be used to avoid
`X-Frame-Option` issues. This behavior can lead to a broken embedding,
which would not look very nice.

Feedback welcome.

_bibliography/papers.bib

@@ -54,9 +54,8 @@ @article{PhysRev.47.777
   altmetric={248277},
   dimensions={true},
   google_scholar_id={qyhmnyLat1gC},
+  video={https://www.youtube-nocookie.com/embed/aqz-KE-bpKQ},
   additional_info={. *More Information* can be [found here](https://github.com/alshedivat/al-folio/)},
-  acceptance={9/123},
-  acceptance_rate={7\%},
   selected={true}
 }
 

_config.yml

@@ -413,6 +413,7 @@ enable_navbar_social: false # enables displaying social links in the navbar on t
 enable_project_categories: true # enables categorization of projects into multiple categories
 enable_medium_zoom: true # enables image zoom feature (as on medium.com)
 enable_progressbar: true # enables a horizontal progress bar linked to the vertical scroll position
+enable_video_embedding: false # enables video embedding for bibtex entries. If false, the button opens the video link in a new window.
 
 # -----------------------------------------------------------------------------
 # Library versions

_layouts/bib.liquid

@@ -199,8 +199,10 @@
           <a href="{{ entry.supp | prepend: '/assets/pdf/' | relative_url }}" class="btn btn-sm z-depth-0" role="button">Supp</a>
         {% endif %}
       {% endif %}
-      {% if entry.video %}
+      {% if entry.video and site.enable_video_embedding %}
         <a class="abstract btn btn-sm z-depth-0" role="button">Video</a>
+      {% elsif entry.video %}
+        <a href="{{ entry.video }}" class="btn btn-sm z-depth-0" role="button">Video</a>
       {% endif %}
       {% if entry.blog %}
         <a href="{{ entry.blog }}" class="btn btn-sm z-depth-0" role="button">Blog</a>
@@ -307,7 +309,7 @@
       </div>
     {% endif %}
 
-    {% if entry.video %}
+    {% if entry.video and site.enable_video_embedding %}
       <!-- Hidden video block -->
       <div class="abstract hidden">
         <div style="text-align: center;">{% include video.liquid path=entry.video class="img-fluid rounded z-depth-1" controls=true %}</div>