apache: Don't redirect to HTTPS for .onion domains

Tests:

- Without the patch, run torsocks curl -kv http://DOMAIN.onion. Observe that
redirection to https happens.

- Without the patch, run curl -kv http://localhost. Observe that redirection to
https happens.

- With the patch, run torsocks curl -kv http://DOMAIN.onion. Observe that
redirection to https does not happen.

- With the patch, run curl -kv http://localhost. Observe that redirection to
https happens.

[sunil: Perform case insensitive match]
[sunil: Remove capture of domain name match]
[sunil: Strictly check that domain ends with .onion]
Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: Sunil Mohan Adapa <sunil@medhas.org>
Tested-by: Sunil Mohan Adapa <sunil@medhas.org>

data/etc/apache2/sites-available/plinth-ssl.conf

@@ -10,6 +10,9 @@
 ##
 <Location /plinth>
     RewriteEngine on
+    # Don't redirect for onion sites as it is not needed and leads to
+    # unnecessary warning.
+    RewriteCond %{HTTP_HOST} !^.*\.onion$ [NC]
     ReWriteCond %{HTTPS} !=on
     RewriteRule ^ https://%{HTTP_HOST}%{REQUEST_URI} [R=301,L]
 </Location>