Initial work on Vision support in text conversations and Image Editing

This is a huge patch which does some major refactoring like:

- renaming "Image Generation" to "Image Creation" in most places,
  to better match its new command (`!bai image create`)

- relocating image creation command (`!bai image` -> `!bai image create`),
  so it wouldn't conflict with the new image editing command (`!bai image edit`)

- introducing a new image editing command (`!bai image edit`), which
  is meant to work only with the OpenAI provider, but doesn't fully work yet
  due to https://github.com/64bit/async-openai/issues/364, though a next patch will fix it

- adding support for reading images off of Matrix conversations and forwarding them to
  text conversations. Works for OpenAI, but not for Anthropic yet
  (requires custom patches) and not for OpenAI-Compat (no support for
  images there)

- relocating some utils around (base64, mime)

src/strings/image_generation.rs

@@ -8,7 +8,7 @@ pub fn guide_how_to_proceed() -> String {
     message.push_str("💡 Respond in this thread with:\n");
     message.push_str("- more messages: to expand on your original prompt\n");
     message.push_str(
-        "- a message saying `again`: to generate one more image with the current prompt.\n",
+        "- a message saying `again`: to generate one more image with the current prompt\n",
     );
 
     message