Re-add ability to exclude posts from search (#3089)

Earlier there was a tag in the `_config.yml` file to exclude the posts
from the search results. I could no longer find that option and hence
modified the `search.liquid.js` file.
It basically checks if the `posts_in_search` tag is true or not.

_config.yml

@@ -52,6 +52,7 @@ navbar_fixed: true
 footer_fixed: true
 search_enabled: true
 socials_in_search: true
+posts_in_search: true
 bib_search: true
 
 # Dimensions

_scripts/search.liquid.js

@@ -52,6 +52,7 @@ ninja.data = [
       {%- endif -%}
     {%- endif -%}
   {%- endfor -%}
+  {%- if site.posts_in_search -%}
     {%- for post in site.posts -%}
       {
         {%- assign title = post.title | escape | strip -%}
@@ -76,6 +77,7 @@ ninja.data = [
         },
       },
     {%- endfor -%}
+  {%- endif -%}
   {%- for collection in site.collections -%}
     {%- if collection.label != 'posts' -%}
       {%- for item in collection.docs -%}