Use Rust for the front-end too (#374)

Rust can compile to WASM, so let's use it for front-end code as well.

To compile the front-end:

```
cargo install cargo-web
cargo web deploy -p plume-front
```

plume-front/Cargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "plume-front"
+version = "0.1.0"
+authors = ["Plume contributors"]
+
+[dependencies]
+stdweb = "0.4"

plume-front/src/main.rs

@@ -0,0 +1,67 @@
+#[macro_use]
+extern crate stdweb;
+
+use stdweb::{unstable::TryFrom, web::{*, event::*}};
+
+fn main() {
+    auto_expand();
+    menu();
+    search();
+}
+
+/// Auto expands the editor when adding text
+fn auto_expand() {
+    match document().query_selector("#plume-editor") {
+        Ok(Some(x)) => HtmlElement::try_from(x).map(|article_content| {
+            let offset = article_content.offset_height() - (article_content.get_bounding_client_rect().get_height() as i32);
+            article_content.add_event_listener(move |_: KeyDownEvent| {
+                let article_content = document().query_selector("#plume-editor").ok();
+                js! {
+                    @{&article_content}.style.height = "auto";
+                    @{&article_content}.style.height = @{&article_content}.scrollHeight - @{offset} + "px";
+                }
+            });
+        }).ok(),
+        _ => None
+    };
+}
+
+/// Toggle menu on mobile device
+///
+/// It should normally be working fine even without this code
+/// But :focus-within is not yet supported by Webkit/Blink
+fn menu() {
+    document().get_element_by_id("menu")
+        .map(|button| {
+            document().get_element_by_id("content")
+                .map(|menu| {
+                    button.add_event_listener(|_: ClickEvent| {
+                        document().get_element_by_id("menu").map(|menu| menu.class_list().add("show"));
+                    });
+                    menu.add_event_listener(|_: ClickEvent| {
+                        document().get_element_by_id("menu").map(|menu| menu.class_list().remove("show"));
+                    });
+                })
+        });
+}
+
+/// Clear the URL of the search page before submitting request
+fn search() {
+    document().get_element_by_id("form")
+        .map(|form| {
+            form.add_event_listener(|_: SubmitEvent| {
+                document().query_selector_all("#form input").map(|inputs| {
+                    for input in inputs {
+                        js! {
+                            if (@{&input}.name === "") {
+                                @{&input}.name = @{&input}.id
+                            }
+                            if (@{&input}.name && !@{&input}.value) {
+                                @{&input}.name = "";
+                            }
+                        }
+                    }
+                }).ok();
+            });
+        });
+}