[FIX] website: make website default language required

The website `default_lang_id` was not required,
while the algorithm choosing the website language
expects it to be required.

This solves the below issue, when your browser language is en_US:
 - add a second language to the website (e.g. fr_BE or es_VE),
 - in the website settings, remove English from the languages
 - in the website settings, unset the default language
 - try to go on the website

You will have a crash:
```
  File "/home/odoo/src/odoo/saas-11/addons/website/models/ir_http.py", line 193, in _dispatch
    resp.set_cookie('website_lang', request.lang)
  File "/usr/lib/python2.7/dist-packages/werkzeug/wrappers.py", line 992, in set_cookie
    self.charset))
  File "/usr/lib/python2.7/dist-packages/werkzeug/http.py", line 905, in dump_cookie
    value = to_bytes(value, charset)
  File "/usr/lib/python2.7/dist-packages/werkzeug/_compat.py", line 106, in to_bytes
    raise TypeError('Expected bytes')
TypeError: Expected bytes
```

This is because the `request.lang` is set to `False`
because of the below algorithm in `ir_http.py`:
```
    nearest_lang = not func and self.get_nearest_lang(path[1])
    url_lang = nearest_lang and path[1]
    preferred_lang = ((cook_lang if cook_lang in langs else False)
                      or self.get_nearest_lang(request.lang)
                      or request.website.default_lang_code)

    is_a_bot = self.is_a_bot()

    request.lang = request.context['lang'] = nearest_lang or preferred_lang
```
`nearest_lang` is `False`, because there is no nearest language
available for the browser lang (`en_US`)
`cook_lang` is `False` for users who never went on the website
`request.website.default_lang_code`, which is the last feedback,
is also `False` because you removed the default language
in the website settings.

opw-695621

addons/website/models/res_config.py

@@ -10,7 +10,7 @@ class website_config_settings(osv.osv_memory):
         'website_name': fields.related('website_id', 'name', type="char", string="Website Name"),
 
         'language_ids': fields.related('website_id', 'language_ids', type='many2many', relation='res.lang', string='Languages'),
-        'default_lang_id': fields.related('website_id', 'default_lang_id', type='many2one', relation='res.lang', string='Default language'),
+        'default_lang_id': fields.related('website_id', 'default_lang_id', type='many2one', relation='res.lang', string='Default language', required=True),
         'default_lang_code': fields.related('website_id', 'default_lang_code', type="char", string="Default language code"),
         'google_analytics_key': fields.related('website_id', 'google_analytics_key', type="char", string='Google Analytics Key'),
         

addons/website/models/website.py

@@ -154,7 +154,7 @@ class website(osv.osv):
         'name': fields.char('Domain'),
         'company_id': fields.many2one('res.company', string="Company"),
         'language_ids': fields.many2many('res.lang', 'website_lang_rel', 'website_id', 'lang_id', 'Languages'),
-        'default_lang_id': fields.many2one('res.lang', string="Default language"),
+        'default_lang_id': fields.many2one('res.lang', string="Default language", required=True),
         'default_lang_code': fields.related('default_lang_id', 'code', type="char", string="Default language code", store=True),
         'social_twitter': fields.char('Twitter Account'),
         'social_facebook': fields.char('Facebook Account'),