Show issue reactions (#421)

```
$ tea issue 230

   #230 issue/pull details: show reactions (open)

  @6543 created 2020-10-22 16:39

  since reactions are utf8 now and most terminals too, we can display them nicely :)

  https://gitea.com/api/v1/repos/gitea/tea/issues/230/reactions

  --------

  1x 🎉  |  1x 👀  |  1x   |  1x 👍  |  1x 👎  |  1x 😆  |  1x 😕  |  1x ❤️
```

caveats:
- reactions are not returned as UTF8 (as was claimed in #230), so they need to be parsed. the library I use doesn't (and can't → ) support all reactions available in gitea
- currently only for issues, as reactions for comments mean an additional API request for each comment..

fixes #230

Co-authored-by: Norwin <git@nroo.de>
Reviewed-on: https://gitea.com/gitea/tea/pulls/421
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Reviewed-by: techknowlogick <techknowlogick@gitea.io>
Co-authored-by: Norwin <noerw@noreply.gitea.io>
Co-committed-by: Norwin <noerw@noreply.gitea.io>

vendor/github.com/enescakir/emoji/emoji.go

@@ -0,0 +1,124 @@
+package emoji
+
+import (
+	"fmt"
+	"strings"
+)
+
+// Base attributes
+const (
+	TonePlaceholder = "@"
+	flagBaseIndex   = '\U0001F1E6' - 'a'
+)
+
+// Skin tone colors
+const (
+	Default     Tone = ""
+	Light       Tone = "\U0001F3FB"
+	MediumLight Tone = "\U0001F3FC"
+	Medium      Tone = "\U0001F3FD"
+	MediumDark  Tone = "\U0001F3FE"
+	Dark        Tone = "\U0001F3FF"
+)
+
+// Emoji defines an emoji object with no skin variations.
+type Emoji string
+
+// String returns string representation of the simple emoji.
+func (e Emoji) String() string {
+	return string(e)
+}
+
+// EmojiWithTone defines an emoji object that has skin tone options.
+type EmojiWithTone struct {
+	oneTonedCode string
+	twoTonedCode string
+	defaultTone  Tone
+}
+
+// newEmojiWithTone constructs a new emoji object that has skin tone options.
+func newEmojiWithTone(codes ...string) EmojiWithTone {
+	if len(codes) == 0 {
+		return EmojiWithTone{}
+	}
+
+	one := codes[0]
+	two := codes[0]
+
+	if len(codes) > 1 {
+		two = codes[1]
+	}
+
+	return EmojiWithTone{
+		oneTonedCode: one,
+		twoTonedCode: two,
+	}
+}
+
+// withDefaultTone sets default tone for an emoji and returns it.
+func (e EmojiWithTone) withDefaultTone(tone string) EmojiWithTone {
+	e.defaultTone = Tone(tone)
+
+	return e
+}
+
+// String returns string representation of the emoji with default skin tone.
+func (e EmojiWithTone) String() string {
+	return strings.ReplaceAll(e.oneTonedCode, TonePlaceholder, e.defaultTone.String())
+}
+
+// Tone returns string representation of the emoji with given skin tone.
+func (e EmojiWithTone) Tone(tones ...Tone) string {
+	// if no tone given, return with default skin tone
+	if len(tones) == 0 {
+		return e.String()
+	}
+
+	str := e.twoTonedCode
+	replaceCount := 1
+
+	// if one tone given or emoji doesn't have twoTonedCode, use oneTonedCode
+	// Also, replace all with one tone
+	if len(tones) == 1 {
+		str = e.oneTonedCode
+		replaceCount = -1
+	}
+
+	// replace tone one by one
+	for _, t := range tones {
+		// use emoji's default tone
+		if t == Default {
+			t = e.defaultTone
+		}
+
+		str = strings.Replace(str, TonePlaceholder, t.String(), replaceCount)
+	}
+
+	return str
+}
+
+// Tone defines skin tone options for emojis.
+type Tone string
+
+// String returns string representation of the skin tone.
+func (t Tone) String() string {
+	return string(t)
+}
+
+// CountryFlag returns a country flag emoji from given country code.
+// Full list of country codes: https://en.wikipedia.org/wiki/ISO_3166-1_alpha-2
+func CountryFlag(code string) (Emoji, error) {
+	if len(code) != 2 {
+		return "", fmt.Errorf("not valid country code: %q", code)
+	}
+
+	code = strings.ToLower(code)
+	flag := countryCodeLetter(code[0]) + countryCodeLetter(code[1])
+
+	return Emoji(flag), nil
+}
+
+// countryCodeLetter shifts given letter byte as flagBaseIndex.
+func countryCodeLetter(l byte) string {
+	return string(rune(l) + flagBaseIndex)
+}