Add storing for Client CA certificates in the database

Add initial support for managing Client Certificate Authority public
certificates as certificate objects in the database. The new provider
type 'clientca' is defined to implement this.

frontend/js/i18n/messages.json

@@ -99,6 +99,7 @@
     "ssl": {
       "letsencrypt": "Let's Encrypt",
       "other": "Custom",
+      "clientca": "Client Certificate Authority",
       "none": "HTTP only",
       "letsencrypt-email": "Email Address for Let's Encrypt",
       "letsencrypt-agree": "I Agree to the <a href=\"{url}\" target=\"_blank\">Let's Encrypt Terms of Service</a>",
@@ -185,7 +186,7 @@
       "title": "SSL Certificates",
       "empty": "There are no SSL Certificates",
       "add": "Add SSL Certificate",
-      "form-title": "Add {provider, select, letsencrypt{Let's Encrypt} other{Custom}} Certificate",
+      "form-title": "Add {provider, select, letsencrypt{Let's Encrypt Certificate} other{Custom Certificate} clientca{Client Certificate Authority}}",
       "delete": "Delete SSL Certificate",
       "delete-confirm": "Are you sure you want to delete this SSL Certificate? Any hosts using it will need to be updated later.",
       "help-title": "SSL Certificates",
@@ -193,6 +194,7 @@
       "other-certificate": "Certificate",
       "other-certificate-key": "Certificate Key",
       "other-intermediate-certificate": "Intermediate Certificate",
+      "clientca-certificate": "Certificate",
       "force-renew": "Renew Now",
       "test-reachability": "Test Server Reachability",
       "reachability-title": "Test Server Reachability",
@@ -231,7 +233,8 @@
       "pass-auth": "Pass Auth to Host",
       "access-add": "Add",
       "auth-add": "Add",
-      "search": "Search Access…"
+      "search": "Search Access…",
+      "client-certificates": "Client Certificates"
     },
     "users": {
       "title": "Users",