i18n: Correctly expand fields in 'format_skeleton'

[tomasr8]

Originally came up here: https://talk.getindico.io/t/japanese-ja-translation-group/542/18

The current behaviour of _adjust_skeleton produces incorrect date formatting for Japanese. This fixes the function to only perform allowed expansions.

More on this: https://cldr-smoke.unicode.org/spec/main/ldml/tr35-dates.html#Matching_Skeletons

Here's the effect this has on the meeting page (event/XXX):

Locale	meeting page before	meeting page after
de	Montag, 20. November	Mo., 20. November
en_CA	Monday, November 20	Mon, November 20
en_GB	Monday, 20 November	Mon, 20 November
en_US	Monday, November 20	Mon, November 20
es	lunes, 20 de noviembre	lun, 20 de noviembre
fr	lundi 20 novembre	lun. 20 novembre
it	lunedì 20 novembre	lun 20 novembre
pl	poniedziałek, 20 listopada	pon., 20 listopada
pt_BR	segunda-feira, 20 de novembro	seg., 20 de novembro
tr	20 Kasım Pazartesi	20 Kasım Pzt
cs	pondělí 20. listopadu	po 20. listopadu
uk	понеділок, 20 листопада	пн, 20 листопада
ja	11月月20日月曜日	11月20日月曜日

The week overview page (category/XXX/overview?period=week) doesn't change except for displaying correctly in Japanese
11月月20日月曜日 -> 11月20日月曜日

FWIW, more recent versions of CLDR do have a specific skeleton for EEEEdMMMM so we'll be able to get the full week day once babel updates.

[ThiefMaster]

I just tested with babel==2.15.0 which uses CLDR 44, but on de_DE I still get Mo., 9. Juli when using format_skeleton('EEEEdMMMM'). I also tried using the native Babel format_skeleton instead of our custom logic and get the same behavior.