Selector menus

[Borketh]

Slash/application commands offer both text input and options menus, as opposed to text commands where it's all text input. This opens up some pretty useful functionality and also can remove some UX issues related to text processing. The first places I see this being used is in docsearch (see #122 ) and in a modified version of linking related servers. Instead of having a bunch of commands and aliases for a command for a specific server, we have one unified "link-discord" command with options we can add on the fly.