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/controller/image/determination/mod.rs

@@ -0,0 +1,18 @@
+use crate::controller::ControllerType;
+mod tests;
+
+pub fn determine_controller(
+    text: &str,
+) -> ControllerType {
+    let text = text.trim();
+
+    if let Some(prompt) = text.strip_prefix(&format!("create")) {
+        return ControllerType::ImageGeneration(prompt.trim().to_owned());
+    }
+
+    if let Some(prompt) = text.strip_prefix(&format!("edit")) {
+        return ControllerType::ImageEdit(prompt.trim().to_owned());
+    }
+
+    ControllerType::UsageHelp
+}