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Add lunr-gr.js and fix lunr-en.js  (#1445)

* Remove Lunr trimmer & bring back colons
* Add Greek Stemmer
* Translate `search_placeholder_text` and `results_found` to Greek

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Round top right corner of code block icon

Reduce visual weight of code blocks

Flatten shadow and remove borders relying on background color to highlight code.

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Fix line number width in code blocks

Code blocks with short line widths can cause `td.gutter` width to stretch, adding in undesired white space.

Declaring widths on the table's columns fixes #1437

Add `{% highlight linenos %}` examples to test and demo sites.

Fix heading

Update Font Awesome link

Update Greedy Navigation source link

Update GitHub Pages installation instructions

Update copyright date

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v2: Introduce show_overlay_excerpt and document  (#1436)

Make excerpts optional in header overlay

Some posts and some header images don't lend themselves well to
displaying the excerpt in the overlay. Make this optional by
introducting a new boolean variable:

`page.header.show_overlay_excerpt`

Set it to implicitly default to `true` so existing users are unaffected by
testing explicitly for "!= false".

This can be enabled globally for a site by changing the default to `false`
in the local _config.yml, or per page by specifying the value in front
matter.

Document show_overlay_excerpt variable

* Add show_overlay_excerpt variable to the Header Overlay field documentation table.

Close #1429 

Release 4.8.1 💎

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Update Lunr (#1419)

* Update Lunr to 2.1.5
* Remove colons from query string

Fixes #1419 

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Add Japanese localized UI text strings