Updated emoji API endpoints

Changed:

• check uniqueness of shortcode globally, not only in pack
• endpoint to get pack.json for certain pack, so on downloading pack no necessary to load all packs from remote instance
• routes renamings and method changes
• creating emoji in pack without shortcode (is taken from filename)

Fixed:

• emoji pack cache (different maps were set on pack caching)
• adding/updating emoji with unique shortcode within pack, but not uniq globally leads to rewriting old emoji in emoji ets table
• fix for doubled path in emoji.txt files on local import
• little fix for instance urls, when url is used with / on the end (e.g. https://fedi.absturztau.be/)
• fix for routes in downloading pack to the local instance
• fix for routes in test mocks

Closes #1545 (closed) Closes #1604 (closed) Closes #1649 (closed)