Add explicit login functionality

web/templates/login.html

@@ -0,0 +1,41 @@
+{{define "content"}}
+{{if .User}}
+<section class="card">
+  <h1>Logged in</h1>
+  <p>You are logged in as <strong>{{.User}}</strong>.</p>
+  <dl>
+    <dt>Maximum size</dt><dd>{{.MaxSize}}</dd>
+    <dt>Longest lifetime</dt><dd>{{.MaxExpiry}}</dd>
+    <dt>Custom names</dt><dd>{{if .Vanity}}allowed{{else}}not allowed{{end}}</dd>
+  </dl>
+  <p class="actions">
+    <a class="button" href="{{.Base}}">Upload a file</a>
+    <form method="post" action="{{.Base}}logout"><button type="submit" class="link">Log out</button></form>
+  </p>
+</section>
+{{else}}
+<section class="card">
+  <h1>Log in</h1>
+  <p class="hint">
+    A token raises your size and lifetime limits and lets you choose custom
+    names. Logging in keeps it for this browser, so you do not have to paste it
+    for every upload. Without one you can still upload anonymously.
+  </p>
+
+  {{if .Error}}<p class="error">{{.Error}}</p>{{end}}
+
+  <form method="post" action="{{.Base}}login">
+    <input type="hidden" name="next" value="{{.Next}}">
+    <label class="field">
+      <span>Token</span>
+      <input type="password" name="token" autocomplete="current-password" required autofocus>
+    </label>
+    <label class="check">
+      <input type="checkbox" name="persist" value="1" checked>
+      <span>Stay logged in on this device</span>
+    </label>
+    <button type="submit">Log in</button>
+  </form>
+</section>
+{{end}}
+{{end}}