Basic frontend

I'm sorry for the big commit

- Added yarte templating engine
- Wrote frontend: all HTML except file input area, basic CSS (themes soon), JS.
No client-side encryption yet. And no working API.
- Simple but handy build system in Makefile
(requires pacman -S entr / apt install entr)

Makefile

@@ -0,0 +1,12 @@
+watch-scss:
+	@sass --watch scss:static
+
+build-scss:
+	@sass scss:static -s compressed
+
+build:
+	make build-scss
+	cargo build --release
+
+dev:
+	@while sleep 0.1; do find ./templates -type f | entr -d -r cargo run && break; done