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/tests.rs

@@ -0,0 +1,37 @@
+#[test]
+fn determine_controller() {
+    struct TestCase {
+        name: &'static str,
+        input: &'static str,
+        expected: super::ControllerType,
+    }
+
+    let test_cases = vec![
+        TestCase {
+            name: "Top-level is usage help",
+            input: "",
+            expected: super::ControllerType::UsageHelp,
+        },
+        TestCase {
+            name: "Top-level with some text is usage help",
+            input: "Some text",
+            expected: super::ControllerType::UsageHelp,
+        },
+        TestCase {
+            name: "Image generation triggered by create prefix",
+            input: "create Some prompt",
+            expected: super::ControllerType::ImageGeneration("Some prompt".to_owned()),
+        },
+
+        TestCase {
+            name: "Image edit triggered by edit prefix",
+            input: "edit Turn this into an anime-style image",
+            expected: super::ControllerType::ImageEdit("Turn this into an anime-style image".to_owned()),
+        },
+    ];
+
+    for test_case in test_cases {
+        let result = super::determine_controller(test_case.input);
+        assert_eq!(result, test_case.expected, "Test case: {}", test_case.name);
+    }
+}