Add auto-completion to /tag

[Zabuzard]

Slashcommands now have autocompletion support.

/tag is the perfect candidate for this and should support it.

Therefore, simply implement onAutoComplete which is already avaialble to SlashCommandAdapter and incorperate existing tags into it.

[Taz03]

same as #367

[Zabuzard]

Right. closing that in favor of this.

[Taz03]

Right. closing that in favor of this.

i'd do this the other way around