Improves a11y of password protected video form

According to the WCAG success criterias 1.3.1 (Info and relationships),
3.3.2 (Labels or Instructions) and 4.1.2 (Name, Role, Value), labels
or instructions are required when content requires user input.

To do so, this commit adds a label for the password input and hide it
with a CSS visually-hidden method to not alter the design.

Resources:
- https://www.w3.org/TR/WCAG21/#labels-or-instructions
- https://www.w3.org/TR/WCAG21/#info-and-relationships
- https://www.w3.org/TR/WCAG21/#name-role-value
- https://www.w3.org/TR/WCAG20-TECHS/H44.html
- https://www.w3.org/WAI/WCAG21/Understanding/labels-or-instructions.html
- https://gomakethings.com/hidden-content-for-better-a11y/

templates/password.tpl

@@ -5,7 +5,8 @@
         <h2>{t}This video is protected{/t}</h2>
         <p>{t}You need a password in order to download this video.{/t}</p>
         <form action="" method="POST">
-            <input class="URLinput" type="password" name="password" title="{t}Video password{/t}" />
+            <label class="sr-only" for="password">{t}Video password{/t}</label>
+            <input class="URLinput" type="password" name="password" id="password" />
             <br/><br/>
             <input class="downloadBtn" type="submit" value="{t}Download{/t}" />
         </form>