Generate useful description for OpenGraph/oEmbed links

[imagejan]

When one-boxing a link to the new website on the forum, e.g. in this post, the description currently reads:

ImageJ is an open source image processing program designed for scientific multidimensional images.

It's possible to specify the description with description: in the front matter, but ideally we should improve the default description to be the same that is also used in the search (see this Gitter discussion).

@ctrueden suggests translating the Python code here:

imagej.github.io/_search/server/index-site.py

Lines 18 to 32 in 2f99613

	def first_sentence(lines):
	def good(line):
	if line.startswith('#'): return False # ignore headers
	if re.match('^[=-]+$', line): return False # ignore section dividers
	return True
	s = ''.join(line for line in lines if good(line))
	s = re.sub('<[^>]*>', '', s) # strip HTML
	s = re.sub('{%[^%]*%}', '', s) # strip Liquid tags
	s = re.sub('\[([^]]*)\]\([^\)]*\)', '\\1', s) # strip Markdown links
	s = re.sub('\*+\\s*([^\*]*)\\s*\*+', '\\1', s) # strip emphasis
	s = re.sub('\\s\\s*', ' ', s) # unify whitespace
	dot = s.find('. ')
	if dot >= 0: s = s[:dot+1] # cut down to first sentence
	return s.strip()

... to Liquid and call it from here:

imagej.github.io/_includes/layout/onebox-info

Lines 25 to 27 in 2f99613

	{%- else -%}
	{{ site.description }}
	{%- endif -%}