Indexing: Restrict indexing to a known list of content types

[m-i-l]

The search results are now restricted to 2 known content_types, i.e. text/html and text/plain now, as per #147 . It would also make sense to restrict indexing to a list of known content_types too. While it might be fine to only show HTML and plain text in results, there are some additional files which might still be useful to retain in the index for additional functionality, so I'd propose restricting indexing to:

text/html,
text/plain,
application/json,
text/xml,
application/rss+xml,
application/xml,
application/xhtml+xml,
application/atom+xml,
application/feed+json

This could be extended, e.g. if #70 is implemented.

It doesn't look like there is an easy hook in scrapy for doing this (e.g. along the lines of allow_domains in LinkExtractor), but it should be possible to implement via a check at the top of customparser in search_my_site_parser.py which returns None (like the existing logic for article type exclusion functionality).

For reference, the full list of currently indexed content types is:

"text/html",91813,
"application/json",1840,
"text/xml",1687,
"application/rss+xml",1532,
"application/xml",1529,
"application/javascript",648,
"text/plain",595,
"application/xhtml+xml",256,
"application/octet-stream",243,
"application/atom+xml",233,
"application/manifest+json",135,
no content type specified,102,
"text/css",89,
"application/feed+json",32,
"text/javascript",26,
"application/pgp-keys",24,
"application/opensearchdescription+xml",20,
"application/x-tex",17,
"text/gemini",17,
"text/x-bibtex",16,
"text/csv",14,
"application/json+oembed",13,
"text/markdown",13,
"application/pgp-signature",12,
"text/calendar",10,
"application/pgp-encrypted",8,
"application/rdf+xml",7,
"text/x-csrc",7,
"text/x-python",6,
"text/x-c",5,
"text/x-diff",5,
"application/stream+json",4,
"application/x-mspublisher",4,
"image/svg+xml",4,
"text/vcard",4,
"text/x-opml+xml",4,
"application/jf2feed+json",3,
"application/rsd+xml",3,
"application/x-httpd-ea-php55",3,
"application/x-sh",3,
"binary/octet-stream",3,
"text/x-c++src",3,
"application/jf2+json",2,
"application/jrd+json",2,
"application/mathematica",2,
"application/pem-certificate-chain",2,
"application/rtf",2,
"application/vnd.wolfram.mathematica.package",2,
"application/x-javascript",2,
"application/x-tcl",2,
"text/x-java-source",2,
"text/x-opml",2,
"text/x-sh",2,
"x-world/x-vrml",2,
"application/mf2+json",1,
"application/opml+xml",1,
"application/pdf",1,
"application/powder+xml",1,
"application/vnd.apple.keynote",1,
"application/x-msdownload",1,
"application/x-perl",1,
"application/x-sql",1,
"application/x-trash",1,
"application/x-x509-ca-cert",1,
"audio/mpegurl",1,
"audio/x-pn-realaudio",1,
"text/prs.fallenstein.rst",1,
"text/vnd.wap.wml",1,
"text/x-java",1,
"text/x-rsrc",1