accessibility: Add `aria-describedby` to login/forgot password (LMS-602)

Also adding the email description to the login page and applying the
same aria attribute to the correspondin input on that page.

From LMS-602:

> There is additional explanatory text, marked up in a span element after
> the email input. The input elements should be explicitly associated with
> this additional text using the aria-describedby attribute. This will
> require an ID to be assigned to the span element.

lms/templates/forgot_password_modal.html

@@ -19,8 +19,8 @@
           <ol class="list-input">
             <li class="field required text" id="field-email">
               <label for="pwd_reset_email">${_("Your E-mail Address")}</label>
-              <input class="" id="pwd_reset_email" type="email" name="email" value="" placeholder="example: username@domain.com" />
-              <span class="tip tip-input">${_("This is the email address you used to register with {platform}").format(platform=settings.PLATFORM_NAME)}</span>
+              <input class="" id="pwd_reset_email" type="email" name="email" value="" placeholder="example: username@domain.com" aria-describedby="pwd_reset_email-tip" />
+              <span class="tip tip-input" id="pwd_reset_email-tip">${_("This is the e-mail address you used to register with {platform}").format(platform=settings.PLATFORM_NAME)}</span>
             </li>
           </ol>
         </fieldset>

lms/templates/login.html

@@ -113,7 +113,8 @@
         <ol class="list-input">
           <li class="field required text" id="field-email">
             <label for="email">${_('E-mail')}</label>
-            <input class="" id="email" type="email" name="email" value="" placeholder="example: username@domain.com" required aria-required="true" />
+            <input class="" id="email" type="email" name="email" value="" placeholder="example: username@domain.com" required aria-required="true" aria-described-by="email-tip" />
+            <span class="tip tip-input" id="email-tip">${_("This is the e-mail address you used to register with {platform}").format(platform=settings.PLATFORM_NAME)}</span>
           </li>
           <li class="field required password" id="field-password">
             <label for="password">${_('Password')}</label>