[FIX] website_slides: prevent exception while accessing embed externa…

…l of slide

Currently, an exception is generated when the user tries to access embed view
of slides by following the steps:
 - Install "eLearning" with a demo data
 - Open external embed view of slide 1 by "/slides/embed_external/1" in url

Error:
`AttributeError: 'slide.channel' object has no attribute 'website_share_url'`

This error is because commit [1] added code that tries to access the
'website_share_url' field in the 'slide.channel' model in template
'embed_slide' , but it actually belongs to 'slide.slide'.

This commit resolved the above issue by accessing "website_share_url"
with "slide" instead of "slide.channel".

[1]-db63c03

sentry-5095823727

closes #159442

X-original-commit: e45456a
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>

addons/website_slides/views/website_slides_templates_lesson_embed.xml

@@ -43,7 +43,7 @@
                             <t t-if="is_external_embed">
                                 <t t-call="website_slides.slide_share_modal">
                                     <t t-set="record" t-value="slide"/>
-                                    <t t-set="website_share_url" t-value="slide.channel_id.website_share_url"/>
+                                    <t t-set="website_share_url" t-value="slide.website_share_url"/>
                                     <t t-set="include_embed" t-value="True"/>
                                     <t t-set="embed_hide_starting_page" t-value="True"/>
                                 </t>