Update release notes for upcoming 1.2.1

Typo (through was misspelled)

[MRG+1] Added documentation about accessing header values

[MRG+1] Better explain middleware orders and processing directions

* Do not interpret non-ASCII bytes in "Location" and percent-encode them

Fixes GH-2321

The idea is to not guess the encoding of "Location" header value
and simply percent-encode non-ASCII bytes,
which should then be re-interpreted correctly by the remote website
in whatever encoding was used originally.

See https://tools.ietf.org/html/rfc3987#section-3.2

This is similar to the changes to safe_url_string in
https://github.com/scrapy/w3lib/pull/45

* Remove unused import

[MRG+1] Added a note about invalid spider names in python 2 (fixes #2251)

Document the effects of the middleware order in a more detailed way.

[MRG+1] Fix JsonWriterPipeline example in docs

Expliciting what actually happens by adding links to the respective methods
that are invoked in each processing phase.

Add more details, making it easier to understand what are the effects of
setting a downloader middleware order.

[MRG+1] Use OpenSSL default ciphers

genspider: removing www. from starturl templates

Twisted default TLS options restricts the ciphers list a bit -- "a secure default"
https://github.com/twisted/twisted/blob/e38cc25a67747899c6984d6ebaa8d3d134799415/src/twisted/internet/_sslverify.py#L1861

We want to be a bit more permissive with Scrapy
(at least as permissive as Scrapy 1.0 was, and which used a default OpenSSL Context)

See https://www.openssl.org/docs/manmaster/apps/ciphers.html#CIPHER_STRINGS

OpenSSL's 'DEFAULT' seems to be reasonable enough.

Fixes #2311

Update release notes for upcoming 1.2.0

Change release section titles to have correct links in ToC

Remove duplicate colons from the feed export settings docs

Update 1.2 release notes with data_path changes from #1581

[MRG+1] Make data_path work when outside project (used by HttpCacheMiddleware and Deltafetch plugin)

[MRG+1] Deprecate official Ubuntu packages and update installation instructions

[MRG+1] architecture docs: restore explanation loop to step 1 (see comment in 3ac3ac4d)