Adds a label to audio options inputs

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 "From" and the "To" inputs
displayed when a user chose to get the audio of a file only.

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

templates/index.tpl

@@ -25,8 +25,8 @@
                         {t}Audio only (MP3){/t}
                     </label>
                     <div class="seekOptions">
-                        {t}From{/t} <input type="text" pattern="(\d+:)?(\d+:)?\d+(\.\d+)?" placeholder="HH:MM:SS" value="" name="from"/>
-                        {t}to{/t} <input type="text" pattern="(\d+:)?(\d+:)?\d+(\.\d+)?" placeholder="HH:MM:SS" value="" name="to"/>
+                        <label id="from">{t}From{/t}</label> <input type="text" pattern="(\d+:)?(\d+:)?\d+(\.\d+)?" placeholder="HH:MM:SS" value="" name="from" id="from" />
+                        <label id="to">{t}to{/t}</label> <input type="text" pattern="(\d+:)?(\d+:)?\d+(\.\d+)?" placeholder="HH:MM:SS" value="" name="to" id="to" />
                     </div>
                 </div>
             </div>