[FIX] hr_contract: Load calendar mismatch style

Commit f17655dd introduced a new file (`views/assets.xml`) in
the manifest to load a new scss file. But it also deleted from
the manifest an existing file (`views/hr_contract_templates.xml`)
without moving the content from removed file to the new file.
Hence, style defined in `calendar_mismatch.scss` was never loaded.

addons/hr_contract/views/assets.xml

@@ -2,6 +2,7 @@
     <template id="assets_backend" inherit_id="web.assets_backend">
         <xpath expr="." position="inside">
             <link rel="stylesheet" type="text/css" href="/hr_contract/static/src/scss/state_selection.scss"/>
+            <link rel="stylesheet" type="text/scss" href="/hr_contract/static/src/css/calendar_mismatch.scss"/>
         </xpath>
     </template>
 </odoo>
\ No newline at end of file

addons/hr_contract/views/hr_contract_templates.xml

-<?xml version="1.0" encoding="utf-8"?>
-<odoo>
-    <template id="assets_backend" name="hr_contract_assets_backend" inherit_id="web.assets_backend">
-        <xpath expr="." position="inside">
-            <link rel="stylesheet" type="text/scss" href="/hr_contract/static/src/css/calendar_mismatch.scss"/>
-        </xpath>
-    </template>
-</odoo>