Convert mailer_emoji JSON file to map

This fixes a pending TODO comment regarding inefficient tags to emojis
mapping, by requiring a full scan over emoji aliases to determine
matches.

Instead, now the JSON file is a map, with aliases as keys, and emojis as
values. The script to convert the file with Python was:

```python
import json

with open("./mailer_emoji.json", "r", encoding="utf-8") as f:
    content = json.load(f)

emoji_map = {}
for emoji in content:
    for alias in emoji["aliases"]:
        if alias in emoji_map:
            print("WARNING: Duplicate alias:", alias)
            continue
        emoji_map[alias] = str(emoji["emoji"])

sorted_emoji_map = {k: emoji_map[k] for k in sorted(emoji_map)}

with open("./mailer_emoji_map.json", "w", encoding="utf-8") as f:
    json.dump(sorted_emoji_map, f, indent=4, ensure_ascii=False)
```

server/smtp_sender.go

@@ -4,14 +4,15 @@ import (
 	_ "embed" // required by go:embed
 	"encoding/json"
 	"fmt"
-	"heckel.io/ntfy/log"
-	"heckel.io/ntfy/util"
 	"mime"
 	"net"
 	"net/smtp"
 	"strings"
 	"sync"
 	"time"
+
+	"heckel.io/ntfy/log"
+	"heckel.io/ntfy/util"
 )
 
 type mailer interface {
@@ -131,31 +132,23 @@ This message was sent by {ip} at {time} via {topicURL}`
 }
 
 var (
-	//go:embed "mailer_emoji.json"
+	//go:embed "mailer_emoji_map.json"
 	emojisJSON string
 )
 
-type emoji struct {
-	Emoji   string   `json:"emoji"`
-	Aliases []string `json:"aliases"`
-}
-
 func toEmojis(tags []string) (emojisOut []string, tagsOut []string, err error) {
-	var emojis []emoji
-	if err = json.Unmarshal([]byte(emojisJSON), &emojis); err != nil {
+	var emojiMap map[string]string
+	if err = json.Unmarshal([]byte(emojisJSON), &emojiMap); err != nil {
 		return nil, nil, err
 	}
 	tagsOut = make([]string, 0)
 	emojisOut = make([]string, 0)
-nextTag:
-	for _, t := range tags { // TODO Super inefficient; we should just create a .json file with a map
-		for _, e := range emojis {
-			if util.Contains(e.Aliases, t) {
-				emojisOut = append(emojisOut, e.Emoji)
-				continue nextTag
-			}
+	for _, t := range tags {
+		if emoji, ok := emojiMap[t]; ok {
+			emojisOut = append(emojisOut, emoji)
+		} else {
+			tagsOut = append(tagsOut, t)
 		}
-		tagsOut = append(tagsOut, t)
 	}
 	return
 }