Support Japanese new era "令和 (Reiwa)"

[Mathieu Trudel-Lapierre]

[Background]
Many packages are affected by the requirement to support the new era "Reiwa" (令和）

[Impact]
Users who run Ubuntu in Japanese.

[Test cases]
== Character maps / font support ==

1) Search for character "SQUARE ERA NAME"
2) Verify that the results include at least "SQUARE ERA NAME HEISEI" and "SQUARE ERA NAME REIWA" (there should also be Syouwa, Taisyou and Meizi), and that the glyphs are readable:

 - SQUARE ERA NAME HEISEI: ㍻
 - SQUARE ERA NAME REIWA: 令和 (in a single glyph)

Display of the Reiwa square glyph is font-specific; it may show simply as a empty square or a square with hex characters. If that is the case, the unicode data supports the new character, but the selected font does not include the new glyph.

== Typing / input methods ==

1) Type in 'heisei'
2) Verify that the input method in use gives you an option "平成", and optionally also the square era glyph.
3) Type in 'reiwa'
4) Verify that the input method in use gives you an option that includes "令和", and possibly also the square era glyph (if supported for Heisei)

5) Type in the following strings, and verify that the options are provided in the input method:
    * "れいわ"(reiwa) => "令和"
    * "れいわ"(reiwa) => "㋿"
    * "２０１８ねん"(2018nenn) => "平成三十年"
    * "２０１９ねん"(2019nenn) => "平成三十一年"
    * "２０１９ねん"(2019nenn) => "令和元年"
    * "２０２０ねん"(2020nenn) => "令和二年"

[Regression potential]
This is a potentially large change as it impacts font display, character sets as well as date conversions. As such, extreme care should be taken to ensure that regressions are avoided, such that dates previous to May 1, 2019 continue to display as before, and dates onward are displayed with the new era symbols. The included test cases account for verifying the continued behavior or previous dates.

[Gunnar Hjalmarsson (gunnarhj)]

Fixed in unstable/eoan AFAICT.

[Gunnar Hjalmarsson (gunnarhj)]

Would like to add that anthy had in practice been demoted to universe at the release of Xenial. A user who installs Japanese in Xenial gets fcitx-mozc and ibus-mozc.

With that said, anthy is in the archive, and some Japanese xenial users are using it.