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)

static/script.js

@@ -0,0 +1,29 @@
+addEventListener('DOMContentLoaded', () => {
+  // show password while user holds button
+  const showPswdBtns = document.getElementsByClassName('show-pswd')
+  for (let el of showPswdBtns) {
+    const inp = document.getElementById(el.dataset.input)
+    el.addEventListener('mousedown', () => inp.type = 'text')
+    el.addEventListener('mouseup', () => inp.type = 'password')
+  }
+
+  // add file input by cloning #file-tmpl-js (see index.hbs)
+  const filesContainer = document.querySelector('#tab-file>.form-main')
+  const fileTemplate = document.querySelector('#file-tmpl-js>div')
+  document.getElementById('add-file').addEventListener('click', () => {
+    filesContainer.append(fileTemplate.cloneNode(true))
+  })
+
+  // make tabs usable from keyboard:
+  // when focused on label, space or enter triggers
+  // click on hidden input[type=radio]
+  const tabLabels = document.querySelectorAll('header>label')
+  for (let el of tabLabels) {
+    const inp = document.getElementById(el.htmlFor)
+    el.addEventListener('keyup', (ev) => {
+      if (ev.key == " " || ev.key == "Enter") {
+        inp.click()
+      }
+    })
+  }
+})