Donation link: default value to searxng.org, can be hidden or custom

Add a new setting: general.donation_url

By default the value is https://docs.searxng.org/donate.html

When the value is false, the link is hidden

When the value is true, the link goes to the infopage donation,
the administrator can create a custom page.

searx/settings_defaults.py

@@ -142,6 +142,7 @@ SCHEMA = {
         'instance_name': SettingsValue(str, 'SearXNG'),
         'privacypolicy_url': SettingsValue((None, False, str), None),
         'contact_url': SettingsValue((None, False, str), None),
+        'donation_url': SettingsValue((bool, str), "https://docs.searxng.org/donate.html"),
         'enable_metrics': SettingsValue(bool, True),
     },
     'brand': {